What have scientists achieved in dealing with floods?

• A、They have prevented many rivers from flooding.

• B、They have reduced the damage caused by floods.

• C、They have learned much about conditions for flooding.

• D、They have found most of the ice damson the river.

55.          

He was late for work this morning because he_____find his key.

• A、mustn't

• B、wouldn't

• C、couldn't   

• D、shouldn't