题目

—Why haven’t you finished your work? —The computer system ________ suddenly when I was searching for information on the Internet. A.broke down                              B.broke out          C.broke up                      D.broke in 答案：Ａ（1）求（1+x+x2+x3）（1-x）7的展开式中x4的系数；（2）求（x+4x-4）4的展开式中的常数项；（3）求（1+x）3+（1+x）4+…+（1+x）50的展开式中x3的系数．