∵ C + C +…+ C =2n=128=27，∴n=7．∴Tr+1= C ?（3x）7-r?(?x 1 2 )r=（-1）r?37-r? C ?x7?r+ r 2 ∴由7- r 2 =4得r=6．∴展开式中x4的系数是：3 C =21．故选D．
