[单选题]
Two boxes of masses m and 2m are connected b(i*nz rcos is2l* gjij/ +ol:7y a rope.3gra)t5w51atq+r 4d0h g keuz (See Fig. 4-4.) If the forward force on the more massive trq5ua gtw a0+ek5hd4)gr13 zbox is F, what is the tension in the connecting rope?
