有几道语法题 不太明白
1。In the US, a primary election is a method__ voters select the nominees for public office.
Key: by which. Why is not "that"
2. The stronger__ magnet, the greater the number of lines of magnetic force.
Key: the
I just wonder if we choose "the", it's no verb in the sentence. How come?
3. A neutron star forms when a star much more massive than the Sun dies and exploded.
Key: C
How to correct C? And why not choose D, which should be explodes as the same as dies?
Thanks! | 
