[Grammar] prefers to read rather than (to) exercise

[Oceanlike]

Is (a) or (b) the correct answer to the following synthesis question? I’m inclined to go for (a) which omits ‘to’.

Question: My sister does not like to exercise. She likes to read instead.
a) My sister prefers (these words are fixed) to read rather than exercise.
b) My sister prefers to read rather than to exercise.

Thank you! :-D

 

[Tdol]

They both work for me.

 

[Oceanlike]

the answer given in the book is (b).
I wonder if there is a grammar rule to it.

 

[Tdol]

I suppose they're looking for a parallel structure, but I think a lot of people would happily use a).