根据中英文提示词完成句子。
吸烟有害健康。
Smoking ______our health.
他一接到母亲去世的消息，就立刻踏上归途。
He started back ______ he got the news of his mother’s death.
我过去常常爬山，可现在没有这样的胆量了。
I ______ do a lot of climbing, but I haven’t the nerve for it now.
你坐火车去要花很多钱，为什么不坐公共汽车呢？
It will cost you to go by train; ______ ?
同学之间好好相处很有必要。
It’s______ each other.