—It's so late. Why not write the report tomorrow?—But I don't know _______ I can do it if not now.
A:why
B:when
C:how
D:where
吞噬细胞、T淋巴细胞、B淋巴细胞在体液免疫感应阶段所起的作用是 ( )
面积是160平方米的长方形,它的长y米,宽x米之间的关系表达式是( )
81的平方根是( )
3×5=15