Tropism is a phenomenon by which

Correct Answer: A

Rationale: Tropism refers to plant growth responses to stimuli (e.g., light or gravity), not branching or day/night length, which relate to other processes like photoperiodism.

Correct Answer: C

Rationale: Helper T cells activate B cells via cytokines, enhancing antibody production, unlike plasma cells (antibody producers), cytotoxic T cells, or macrophages.

Correct Answer: B

Rationale: C3 is central to all pathways (classical, lectin, alternative), cleaved to initiate downstream effects, unlike C9 (MAC), C1q (classical), or C4 (classical/lectin).

Correct Answer: A

Rationale: The antigen-binding site is formed by variable regions of both heavy and light chains, creating specificity.

Correct Answer: D

Rationale: Superantigens nonspecifically overactivate T cells via MHC-TCR bridging, evading specific responses.