In 1954 Dr. Jonas Salk began______a killed-virus vaccine for polio.

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问题 In 1954 Dr. Jonas Salk began______a killed-virus vaccine for polio.

选项 A、tested
B、had tested
C、test
D、testing

答案D

解析 The verb began needs either an infinitive or a gerund as its direct object; testing is a gerund.

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