Who was the first scientist to observe bacteria using a single-lens microscope? MCQ with Answer and Explanation

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.

Explanation:
Progesterone stimulates the development of alveolar tissue of the breasts during pregnancy. Estrogen promotes duct development. Prolactin initiates milk synthesis, oxytocin ejection.

Explanation:
Paramecium is a unicellular ciliated protozoan found in freshwater. Earthworm and Hydra are multicellular animals, mushroom is a multicellular fungus. Unicellular organisms carry out all life processes within a single cell.