557.Scientists are working _____ time in order to find a cure for bird flu as soon as possible.

A.on

B.ahead of

C.around

D.against

558.After the robbery,the shop fixed an alarm system ______ further losses.

A.for

B.against

C.from

D.to

557.答案是D;

against time 与时间竞争；ahead of time 提前。

558.答案是B;

考查介词在具体语境中的运用。句意:抢劫俺发生后，这家店按照了报警系统以防更多的损伤。此处against意为:防备，为…而准备。