以下煤气中最容易使人中毒的是（）。
A、高炉煤气
B、焦炉煤气
C、混合煤气
D、转炉煤气
参考答案：D
The man we followed suddenly stopped and looked as if ______ whether he was going in the right
direction. [ ]
A. seeing
B. haying seen
C. to have seen
D. to see
天文学对古代中国有着甚么的意义呢？
