When a and t=0, the magnetic flux of the coil is zero, and the cd edge cuts the magnetic induction line vertically, and the induced electromotive force is the largest, which is: Em=NBSω=BLDω, so A is wrong;

B, turn 90, cd edge speed direction is parallel to the magnetic induction line, so the induced electromotive force of the coil is zero, so B is correct;

C, D, the average induced electromotive force of the coil in the process of rotating 90 is:

. E = n △φ△t = 1× BLD π 2ω = 2ω BLD π, so c is wrong and d is right;

So: BD.