I have bought three ballpoint pens,none of _____writes smoothly.

问题描述:

I have bought three ballpoint pens,none of _____writes smoothly.

which
两个句子用逗号隔开,一主一从;第二个是非限制性定语从句,which代替先行词three ballpoint pens,构成none of which结构,在从句中作主语.
若要填them,逗号需换成句号,改为两个并列句.