The pacemaker of the human heart is the: MCQ with Answer and Explanation

Explanation:
Cyanobacteria (blue-green algae) are photosynthetic, oxygenic prokaryotes that play a crucial role in nitrogen fixation in aquatic and terrestrial ecosystems. Mycorrhiza is a mutualistic symbiotic association between a fungus and the roots of a vascular plant, where the fungus aids in nutrient absorption and the plant provides carbohydrates. Nitrosomonas is a nitrifying bacterium that converts ammonia to nitrite; denitrifying bacteria like Pseudomonas convert nitrates to nitrogen gas. Thus, statements 1 and 2 are correct.

Explanation:
The water vascular system is a unique network of fluid-filled canals found exclusively in Echinodermata (e.g., starfish, sea urchins). It is primarily used for locomotion (via tube feet), capturing and handling food, and respiration. Adult echinoderms exhibit radial symmetry, while their larvae are bilaterally symmetrical.

Explanation:
Potato is a modified stem (tuber) that stores starch. The 'eyes' on a potato are axillary buds, confirming it as a stem. Sweet potato is a modified root (tuberous root), carrot is a taproot, radish is a modified root. Other modified stems include ginger (rhizome), onion (bulb, though bulb is a modified stem with fleshy leaf bases), and garlic.