Assertion (A): The SA node is called the pacemaker of the heart. Reason (R): It generates electrical impulses that initiate the heartbeat. MCQ with Answer and Explanation

Explanation:
Biofertilizers contain living microorganisms like bacteria, cyanobacteria, and fungi that increase nutrient availability (e.g., nitrogen fixation, phosphorus solubilization) for plants. They are eco-friendly and sustainable. Chemical fertilizers are synthetic inorganic salts. Biopesticides control pests. Examples: Rhizobium, Azotobacter, mycorrhiza. Biofertilizers improve soil health without chemical residue.

Explanation:
Alveoli are tiny, thin-walled air sacs surrounded by pulmonary capillaries. Oxygen diffuses across the alveolar and capillary walls into the blood, while CO₂ diffuses out. The trachea, bronchi, and nasal cavity are air passages that do not participate directly in gas exchange.

Explanation:
Digestion converts complex, insoluble food molecules into simple, soluble forms that can be absorbed. It involves mechanical action (chewing, churning) and enzymatic hydrolysis. Absorption is the uptake of nutrients, assimilation is incorporation into body tissues, egestion is elimination of undigested waste.