若|cosa|=-cosa,则x取值范围
A.2kπ≤x≤2kπ+π/2(k∈Z)
B.2kπ+π/2≤x≤2kπ+3π/2(k∈Z)
C.2kπ+3π/2≤x≤2kπ+2π(k∈Z)
D.2kπ+π≤x≤2kπ+3π/2(k∈Z)
最好有原因