when and where的题问句谓语动词是单数还是复数?

when and where的题问句谓语动词是单数还是复数?
看文章,做题的时候既看到有单数又有复数,有点纠结,什么时候要单数,什么时候用复数?比如We will hold an English contest,but when and where ( )yet A hasn't been decided B haven't been decided
我老觉的是单数,但就是不懂为什么,还有最好告诉我什么时候用复数,有一个例句是最好啦