50. A man escaped from the prison last night. It was a long time _A_ the guards discovered what had happened.

　　A before B until C since D when

　　It + 系动词 + 一段时间 + before引导的句子 在…之前花费多少时间。

　　54. Scientists say it may be five or ten years _D_ it is possible to test this medicine on human patients.

　　A since B when C after D before

　　51. In the experiment we kept a watchful eye _D_ the developments and recorded every detail. [keep a watchful eye on 密切主意，留神]

　　A in B at C for D on

　　52. There‘s little chance that mankind would _D_ a nuclear war.

　　A retain B endure C maintain D survive

　　retain vt. 保持，保留; endure v. 忍受; maintain vt. 维持，保持，坚持认为;

　　survive vi. 生存; vt. 活过 … (宾语为某种灾难)。 survive a flood 活过一场洪水。

　　前缀sur表示过 … ，外，超; vive 表示生活，强调活着。

　　54. They usually have less money at the end of the month than _C_ at the beginning.

　　A which is B which was C they have D it is

　　less … than句子前后要平衡结构。

　　55. In the course of a day students do far more than just _A_ classes.

　　A attend B attended C to attend D attending

　　far more than 远远多于，远不止于;也要句子前后平衡结构。

　　56. The French pianist [n. 钢琴师] who had been praised very highly _C_ to be a great disappointment.

　　A turned up B turned in C turned out D turned down

　　turn up 后面不加宾语表示出现，后面加宾语表示将声音调高，调大;

　　turn in + 宾语 上缴，交出; turn down 将声音调低，调小;拒绝;

　　refuse sb. 指直接的回绝; turn sb. down 则指委婉的拒绝;

　　turn out + 动词不定式 最终证明是，结果是。

　　57. Many difficulties have _B_ as a result of the change over to a new type of fuel.

　　A risen B arisen C raised D arrived

　　因本句未加宾语，估不应选不及物动词，可排除C项; arrive vi. 到达，抵达;

　　rise vi. 升起，升高(侧重指旗帜或太阳升起);数量的增高(侧重指水位、价位);

　　the sun rises in the east and sets in the west 太阳东升西落。

　　arise vi. 升起，升高，出现，发生(经常与抽象名词联用)。

　　69. A completely new situation will _B_ when the examination system comes into existence.

　　A arise B rise C raise D arouse

　　33. Last year the advertising rate _D_ by 20 percent.

　　A raised B aroused C arose D rose

　　58. He made such a _D_ contribution to the university that they are naming one of the new buildings after him.

　　A genuine B minimum C modest D generous

　　name after 以…的名字来命名。genuine adj. 真正的，货真价实的; genuine leather 真皮;

　　minimum adj. 最低的，最小的; modest adj. 谦虚谨慎的，适度的;

　　generous adj. 慷慨的，慷慨大方的(修饰贡献等用这个)。

　　59. Helen was much kinder to her youngest child than she was to the others， _D_， of course， made the others jealous.

　　A who B that C what D which