Correct Answer: A

Rationale: The period of a wave is defined as the time it takes for one complete wave cycle to pass a point. It is a crucial parameter in wave analysis and is typically measured in seconds. The period is directly related to the frequency of the wave, as they are reciprocals of each other. Therefore, the correct answer is the time it takes for one complete wave cycle to pass a point (choice A). The period is not related to the number of waves passing a point per unit time (choice C), the distance between two adjacent crests or troughs (choice B), or the maximum displacement of particles in a medium due to the wave (choice D).

Correct Answer: A

Rationale: Terminal velocity in physics refers to the maximum velocity achieved by an object in free fall when the force of gravity equals the force of air resistance. At terminal velocity, the object stops accelerating and maintains a constant speed. This occurs when the opposing forces are balanced, leading to no further increase in speed. Choice B is incorrect as velocity when the object is at rest is zero, not at terminal velocity. Choice C is incorrect as instantaneous velocity refers to the velocity at a specific moment in time, not the maximum speed reached in free fall. Choice D is incorrect because terminal velocity is not exclusive to heavy objects; all objects in free fall can reach terminal velocity under the right conditions.

Correct Answer: A

Rationale: The diaphragm is the primary muscle responsible for inhalation during normal breathing. When it contracts, it flattens, increasing the volume of the thoracic cavity and creating a negative pressure that allows air to flow into the lungs. Intercostal muscles also play a role in expanding the chest cavity during inhalation, but the diaphragm is the main driver of the process. Abdominal muscles are primarily involved in exhalation by pushing the diaphragm upward to expel air from the lungs. Pectoral muscles are involved in movements of the arms and shoulders, not in breathing, making them incorrect choices for this question.

Correct Answer: A

Rationale: Work is defined as the transfer of energy that results in an object's displacement. When work is done on an object, it gains kinetic energy. This means that work is the cause of kinetic energy, as the energy transferred through work leads to the object's motion, which is represented by kinetic energy. Therefore, choice A is correct. Choice B is incorrect because kinetic energy is the result of work, not the other way around. Choice C is incorrect as work and kinetic energy are not equivalent but rather interconnected. Choice D is incorrect as work and kinetic energy are not independent; work leads to changes in kinetic energy.

Correct Answer: D

Rationale: The phospholipid bilayer is a crucial component of the cell membrane, surrounding the cell and regulating the passage of materials into and out of the cell. Its structure enables it to be selectively permeable, controlling which substances can pass through. DNA, proteins, and carbohydrates are essential molecules in the cell but do not directly govern the passage of materials like the phospholipid bilayer does. Therefore, the correct answer is the phospholipid bilayer.