油菜花而不实是缺什么元素。（）
A、Cu
B、Mn
C、Zn
D、B
参考答案：D
–Where is Tom? --He _______ the USA. He _______ back in two months.
A. has gone to; comes B. has bee to; will be
C. has been to; comes D. has gone to; will be
钠离子交换器主要去除水中（）。
A、悬浮物
B、硬度
C、碱度
