Correct Answer: B

Rationale: The skeletal system consists of bones, cartilage, ligaments, and tendons that work together to provide structure, support, protection for internal organs, and facilitate movement. It is responsible for supporting the body's framework and protecting vital organs. The muscular system (option A) consists of muscles that work with the skeletal system to produce movement but does not include bones, cartilage, ligaments, and tendons. The nervous system (option C) is responsible for transmitting signals throughout the body to control movement and other functions. The endocrine system (option D) is a collection of glands that produce hormones to regulate various bodily functions but is not primarily involved in structure, support, and movement as the skeletal system is.

Correct Answer: C

Rationale: Osteoporosis is a condition characterized by a loss of bone density, leading to fragile bones and an increased risk of fractures. Osteoarthritis is a degenerative joint disease affecting cartilage, Rickets is caused by a deficiency in vitamin D leading to impaired bone development, and Scoliosis is an abnormal sideways curvature of the spine. Therefore, the correct answer is osteoporosis as it specifically pertains to decreased bone density and fracture risk, distinguishing it from the other conditions listed.

Correct Answer: D

Rationale: The correct answer is D, 'Boiling point elevation.' Boiling point elevation is a colligative property that describes the increase in the boiling point of a solvent when a non-volatile solute is added to it. This occurs because the presence of solute particles in the solvent lowers the vapor pressure of the solution, requiring a higher temperature to reach the same vapor pressure as the pure solvent. Choices A, B, and C are incorrect. Freezing point depression refers to the lowering of the freezing point of a solution, vapor pressure lowering is the reduction in vapor pressure due to the presence of solute particles, and osmotic pressure is the pressure required to prevent the flow of solvent molecules across a semipermeable membrane in osmosis.

Correct Answer: D

Rationale: Keystone species play a crucial role in maintaining the balance and structure of an ecosystem due to their significant influence. If a keystone species is removed, it disrupts the delicate food web dynamics and can trigger cascading effects throughout the ecosystem. The disruption in predator-prey relationships can lead to population declines and even extinctions of other species. Options A, B, and C are incorrect because the removal of a keystone species would not decrease competition among other species, cause a slight increase in primary productivity, or have a minimal impact on the overall ecosystem structure. Instead, it would have a profound impact, disrupting the food web and causing cascading effects on other populations.

Correct Answer: A

Rationale: According to Newton's second law of motion, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This relationship is mathematically expressed as F = ma, where F represents force, m represents mass, and a represents acceleration. Therefore, an increase in force will result in a proportional increase in acceleration, supporting the statement that force is directly proportional to acceleration. Choice B is incorrect as it suggests an inverse relationship, which is not consistent with Newton's second law. Choice C is incorrect because force and acceleration are indeed related as per Newton's second law. Choice D is incorrect as force can cause acceleration or deceleration depending on the direction of the force relative to the motion of the object, but it does not exclusively cause deceleration.