The animal that has a closed circulatory system is: MCQ with Answer and Explanation

Explanation:
Cyanobacteria (blue-green algae) contain chlorophyll a and phycobilins, mainly phycocyanin (blue) and phycoerythrin (red), hence blue-green color.

Explanation:
Luteinizing hormone (LH) binds to Leydig cells in the testes, stimulating testosterone production. FSH stimulates Sertoli cells for spermatogenesis. GnRH is hypothalamic.

Explanation:
The Sinoatrial (SA) node, located in the right atrium, is the natural pacemaker because its cells are autorhythmic; they can spontaneously generate action potentials without external neural input. Crucially, the SA node has the highest intrinsic rate of depolarization (about 70-80 beats per minute) compared to other autorhythmic tissues like the AV node or Purkinje fibers. Therefore, it sets the pace for the entire heart, overriding slower pacemakers.