I asked him to go to the art exhibition, but he said he had _______seen it.（）

• A、still

• B、already

• C、also

• D、often

53.

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.