A is right

I think they are both correct but have different meanings. Does anyone have any explanations?

Answer A is correct, because B has a "split infinitive."

Any time you put the word "to" next to a verb stem, you have an infinitive:
to jump
to run
to be
to read

One of the more arbitrary rules in English grammar states that we are not allowed to separate the "to" from the verb.

This principle can lead to some conversationally awkward, yet grammatically correct, sentences:
"I promise never to watch that movie again."