There are so many kinds of pens,I can not decide( )

语法来说两个都对，但单项选择就是这，只有一个更合适的

当然是C了
原因：这里不是上面说的宾语从句，而是意思上相当于一个宾语从句的“疑问词+不定式”做宾语
再比如：I don't know what to write(我不知道要写什么)

There are so many kinds of pens,I can not decide which one to choose.
A.to choose which B.to choose which one
C.which one to choose
decide to do sth
decide what to do sth
decide which one to choose
to choose which 中式英语
to choose which one 中式英语

I can not decide which one to choose.这是相当于由which引导的小宾语从句。
I DO STH, which one to choose=STH.
如果选B的话，which就不是引导词，句子里面就有两个动词谓语了，decide和choose。
如果想用B的方式表达，那么句子应该修改为 I can not decide to choose...