句子You'd better后面不应该有to, 其次这题B 和 C都是可以的。传统上认为现行词是人的时候不用that,只用who, 但现代英语无论语法书还是实际应用, that和who都是可以的,who用的更多一些。
福勒现代英语用法里对此问题这样解释:
Fowler's Modern English Usage says: ‘That can also replace who (or whom), especially when the reference is non-specific, as in The person that I saw was definitely a woman.’
这是一道定语从句题,先行词为someone 是人,所以用who 不可以用that
其实这个句子是You'd better to choose someone who is kind and friendly to help you. you think是插入语。