He is busy preparing for the speech,so he can't help
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此处填 ：to do
比较以下结构：can’t help to do sth = 不能帮助做某事
can’t help doing sth = 禁不住做某事,情不自禁地做某事
请看例句：I can’t help to do it.我不能帮忙做这事.He couldn’t help to wash the clothes.我不能帮忙洗衣服.She couldn’t help smiling.她禁不住笑了起来.She couldn’t help envying Helen.她不由得 不妒忌海伦.We cannot help being impressed by their zeal.我们不由得不被他们的热情感动.英语有时还可说can’t help somebody doing something不能使某人不做某事.如：I couldn’t help him saying that.我不能让他不这样说.We couldn’t help them seeing us.我们无法让他们不看到我们.She could not help the tears of rage rolling down her face.她止不住她愤怒的眼泪顺着面颊流下来.
又如下面一题,答案也是 B：She can’t help _________ the house because she’s busy making a cake.A.cleaning B.to clean C.cleaned D.being cleaned
再请看以下试题：
While shopping,people sometimes can’t help _________ into buying something they don’t really need.
A.to persuade B.persuading C.being persuaded D.be persuaded
此题应选C,句中的 can’t help 意为“禁不住”（注意根据句意用被动形式）.
======以下答案可供参考======
供参考答案1:
to do供参考答案2:
嗯，是 to do.