the drug has had immediate effect on patient 前面的为什么是has 而后面的为什么是had?

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• the drug has had immediate effect on patient 前面的为什么是has 而后面的为什么是had?谢谢赐个教个 在下 自学英文中 此题不甚解 虽然没有分数 交流知识 感激之
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