First define the posterior product (define the minimum value and the maximum value) and draw a straight line within the limited range. Hand in the lower (left) curve first, then the right curve, the lower limit first, and then the upper limit.

Here we need to exchange the order of integrals, fix y and draw a straight line from left to right.

That is, when x is between π/2 and π, the lower limit is y=sinx=sin(π-x), and π-x is between 0 and π/2. X=π-arcsiny。

Obviously, the upper limit is π.

So the answer is B.

You chose D because you took it for granted. There is no straight line x=π/2 in the title. You drew it yourself. Where is the lower limit equal to π/2? According to the formula, the lower limit is the curve that passes for the first time, and the upper limit here is a definite value.