Which muscle extends the forearm?

Correct Answer: B

Rationale: Triceps brachii extends the forearm, straightening the elbow, with its three heads inserting on the ulna. Biceps brachii flexes it. Brachialis flexes beneath biceps. Deltoid abducts the arm, not elbow-focused. Triceps' extension role distinguishes it, essential for pushing, unlike flexors or shoulder movers.

Correct Answer: D

Rationale: Animals are heterotrophs, obtaining nutrients by consuming organic matter. Bread mold, a fungus, also functions heterotrophically, secreting enzymes to break down external organic substrates like bread, absorbing the products. Seaweed, oak trees, and grass are autotrophs, using photosynthesis to synthesize food from sunlight. Bread mold's reliance on external digestion and absorption mirrors animal nutrition, unlike plants' self-production, making it the closest match in nutritive strategy, reflecting shared ecological roles as decomposers or consumers.

Correct Answer: D

Rationale: During pregnancy, the placenta forms from maternal and fetal tissues, connecting the embryo to the uterine wall. It transfers oxygen and nutrients from the mother's blood to the fetus's, removes waste, and produces hormones like progesterone to sustain pregnancy. The amnion encases the fetus in fluid, the yolk sac provides early nutrients in other species, and the fallopian tube transports eggs. The placenta's nourishing role, critical for fetal development, distinguishes it as the key gestational organ in humans.

Correct Answer: A

Rationale: Somatic motor (efferent) neurons, part of the voluntary nervous system, release acetylcholine at neuromuscular junctions to initiate skeletal muscle contraction, driving movements like lifting. Sympathetic neurons regulate involuntary functions (e.g., heart rate), not skeletal muscle. Sensory neurons transmit stimuli to the CNS, not activating muscles. 'Muscle neuron' isn't a term neurons innervate, not reside in, muscle. Somatic motor neurons' direct, voluntary control distinguishes them, essential for skeletal muscle's deliberate action, unlike autonomic or sensory roles.

Correct Answer: B

Rationale: Contraction of many sarcomeres, via actin-myosin sliding, shortens the myofibril, a bundle of sarcomeres within a fiber, reducing overall muscle length. Thick filaments (myosin) don't shorten actin slides over them. Motor proteins (myosin) drive this, not shortening themselves. Sarcoplasmic reticulum releases Ca²⁺, not contracting. Myofibril shortening aggregates sarcomere action, distinguishing it, key to muscle movement.