设等差数列{an}的前n项和为Sn，若a1=-11，a3+a7=-6，则当Sn取最小值时，n等于[ ]
A.9
B.8
C.7
D.6
答案：D
分析还原制备钨粉的原理和钨粉颗粒长大的因素。
—Do you think it wise for Chinese mothers to try to do everything for their children?
—No, that’s _____ they are mistaken.[ ]
A. where
B. when
C. in which
D. what
