A:若AB杆计重力,而BC杆不计重力时,由B指向C
B:若AB杆计重力,而BC杆不计重力时,由C指向B
C:若AB杆不计重力,而BC杆计重力时,由B指向A
D:若AB杆不计重力,而BC杆计重力时,由A指向B