why是引导定语从句的关系副词,which是关系代词,定语从句后面该用关系副词还是关系代词,这样来判断,当先行词是reason,我们有三个后选项：why,which/that.再判断关系词在从句中的成分,当关系词在从句中充当主语或宾语时,用which/that,当先行词在从句中充当状语时用why引导.此句中从句he gave后缺少宾语,所以用which充当gave的宾语.
第二个空在be动词之后是表语从句,表语从句可有what 和that引导,what引导时,它在句子中充当成分,而that引导时不充当成分,仅作引导词,此句表语从句中主谓宾都在不缺少成分,所以用that.