题目

—Cross the road carefully, ______ you will keep yourself safe.    —OK, I will. A. or                                        B. and                          C. but     答案：B（2012•江苏一模）选修4-5：不等式选讲已知a1，a2…an都是正数，且a1•a2…an=1，求证：(2+a1)(2+a2)…(2+an)≥3n．