How is polyspermy avoided in humans?

How is sex determined in human beings?

How self - pollination is avoided in Abutilon ?

How is urea formed in the human body? (OR) We are not consuming urea. But in our body urea is produced. Why?

How many pairs of spinal nerve are found in human ?

How does alchol act in human body?

Normally polyspermy does not occur. a. What happens if it occurs? b. Which structure prevents polyspermy in ovum?