The human chromosome number (2n) is: MCQ with Answer and Explanation

Explanation:
Testosterone, produced by Leydig cells in the testes, promotes development of male characteristics like facial hair, deepening of voice, and muscle growth. It also regulates spermatogenesis.

Explanation:
The sinoatrial (SA) node, located in the right atrium, generates electrical impulses spontaneously and initiates each heartbeat, setting the rhythm. It is called the natural pacemaker. The atrioventricular (AV) node delays the impulse before it travels to the ventricles. The bundle of His and Purkinje fibers conduct the impulse through the ventricles.

Explanation:
Stomata are microscopic pores typically on the lower epidermis of leaves, bounded by two guard cells that regulate opening and closing for gas exchange (CO₂ in, O₂ out) and transpiration. Lenticels are on bark, hydathodes release water droplets (guttation), sclereids are thick-walled sclerenchyma cells. Stomatal density varies with environmental conditions.