B. The effective value of the primary coil voltage is 220V V. According to the fact that the voltage is proportional to the number of turns, it is known that the voltage representation is 40V, and B is wrong;
C, from b, I=UR=40 10=4A, c is correct;
D. When the sliding contact P moves to the A end, R becomes larger and the voltage remains unchanged, so the primary and secondary coil current decreases and D is wrong;
So choose AC.