单项选择题

This line was carrying equal number of eastbound and westbound trains, and they _________ regularly.

A．altered
B．alternated
C．switched
D．exchanged
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单项选择题
The explosion in the mine was by a careless miner who lit a match.
A．triggered
B．claimed
C．hampered
D．protested
单项选择题
If you keep on taking on more work than you can do, your health will _________.
A．decline
B．degrade
C．degenerate
D．deteriorate
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