下列各组金属中，可用观察方法区别开的一组是[ ]

A．铁和锌

B．铝和银

C．铁和铝

D．铜和银

Mr. Faugel was convinced that student nervousness had affected their scores; to reduce the anxiety of these students who had already been tested, he gave 22 of them a beta blocker before readministration of the test. Their scores improved significantly. The other 8 students (who did not receive the beta blockers) improved only slightly. Second-time test-takers nationwide had average improvements which were similar to those in Faugel’’s non-beta blocker group. Beta blockers are prescription drugs which have been around for 25 years. These medications, which interfere with the effects of adrenalin, have been used for heart conditions and for minor stress such as stage fright. Now they are used for test anxiety. These drugs seem to help test-takers who have low scores because of test fright, but not those who do not know the material. Since there can be side effects from these beta blockers, physicians are not ready to prescribe them routinely for all test-takers.

Why are beta blockers not prescribed regularly

A．Students are expected to do poorly.

B．There are side effects.

C．The drugs are only 25 years old.

D．They cause test anxiety.