对于关系模式R（X，Y，Z)，下列结论错误的是（76)。A．若X→Y，Y→Z，则X→ZB．若X→Y，X→Z，则X→YZC．若X→Z，则XY→

A、remove

B、been removed

C、be removing

D、be removed

All foreign troops must withdraw from the country.

A．retain

B．retire

C．revise

D．retreat

______ safety, you must keep all medicines away from children.

A．For the sake of

B．In the event of

C．On account of

D．With reference to

an illusion of protection. Quantities of such chemicals, already produced, exist as coolants in millions of refrigerators. When they reach the o-zone layer in the atmosphere, their action cannot be halted. So there is no way to prevent these chemicals from damaging the ozone layer further. Which of the following, if true, most seriously weakens the argument above？

A．It is impossible to measure with accuracy the quantity of ozone-destroying chemicals that exist as coolants in refrigerators.

B．In modern societies, refrigeration of food is necessary to prevent unhealthy and potentially life-threatening conditions.

C．Replacement chemicals that will not destroy ozone have not yet been developed and would be more expensive than the chemicals now used as coolants in refrigerators.

D．Even if people should give up the use of refrigeration, the coolants already in existing refrigerators are a threat to atmospheric ozone.

E．The coolants in refrigerators can be fully recovered at the end of the useful life of the refrigerators and reused.

All man must die, but death can vary ________its significance.

A. with B. in

C. from D. on

A．A.prevent…from spreading

B．B.prevent…to spread

C．C.protect…from spreading

D．D.protect…to spread

19：What must all students show at enrolment？

A.A letter of acceptance from their faculty.

B.Student cards.

C.Their passports.

D.Proof of level of English proficiency.