Reproduction methods

The following are patterns that scientists have observed about reproduction in some animals:
 

  • Oysters release millions of eggs and sperm into the water and fertilisation occurs in the sea.
  • The female mallard duck makes a nest and lays 9-11 fertilised eggs in it.
  • The female elephant is usually pregnant for 22 months, and then gives birth to a single calf.

a)

Why do oysters release such large numbers of eggs?

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b)
What is the advantage of having the elephant calf develop inside the mother before being born?

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c)
Why do many birds, like the mallard duck, lay eggs in groups?

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d) In which of the above animals does external fertilisation of the egg occur?

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