题目

去括号a2-2(ab-b2)-b2的值是（     ） A.a2-2ab        B.a2-2ab-3b2          C.a2-2ab+b2        D.a2-2ab+2b2 答案：CUpon reaching an/a ______  age, children are encouraged, but not forced, to “leave the nest”.  A．appropriateB．potentialC．permanentD．automatic