问题描述：
At the start of this “Cancer Crusade,” researchers were already tackling some tough questions……问were already tackling 为什么要用过去进行时,还有加个already,为啥不用过去完成时或一般过去时;还有tackle不是阻止的意思吗?怎么在这儿变解决了?
问题解答：
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