Circle 1 correct, (20 15! ! )(20 16! ! )=(20 15*20 13*。 . . . *3* 1)(20 16*20 14*。 . . *4*2)=20 16*20 15*。 . . *3*2* 1，(n！ ! ) is called double factorial (n! ) This is called factorial. 2 pairs of circles, disassemble, 3 pairs of circles, 4 pairs of circles, choose D.