gas exchange

[shal3]

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The membrane that functions in respiration in the
embryo is the

A. amnion.
B. allantois.
C. chorion.
D. umbilical cord.​

E. yolk sac

Kaplan says the answer is B. but chorion also plays a role in gas exchange. So does allantois play a bigger role then?

Thanks

 

[LiteBear2011]

The chorion is largely responsible for the exchange of nutrients (see Wikipedia article). The chorionic villi invades the endometrium and allows for the exchange of nutrients from mother to child.
The allantois is responsible for the exchange of gases (see Wikipedia article)