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.