(to run/running/run) in winter because it helps them to keep warm

(to run/running/run) in winter because it helps them to keep warm
(to run/running/run) in winter because it helps them to keep warm

(to run/running/run) in winter because it helps them to keep warm
这是一道选择题吧?答案应该是run,因为这样一来就构成了一个祈使句,不过最好在because前加一个逗号.

to run ,不定式做主语