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!

1.by which表方式。that在定语从句中要作成分，在这里主谓宾已经全了。

2.the more...the more

3.d-->explodes