A) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
B) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
C) If the assertion is true but the reason is false
D) If both the assertion and reason are false
E) If the assertion is false but reason is true
Correct Answer: A
Solution :
In Amphioxus, some exchange of
and 
occurs between the water current and blood through the gill-clefts, but this appears doubtful since the blood contains no respiratory pigment. The pharyngeal wall of Branchiostoma is richly vascular and the water current entering the pharyngeal cavity brings 
The blood flows so close to the surface that some exchange between of blood and of water can easily occur. It appears more probable that an exchange of gases occurs over the whole surface of the body and particularly in the walls of atrium.
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