What is(are) the product(s) in the Pd-catalyzed hydrogenation of 1,2-dimethylcyclopentene?

What is(are) the product(s) in the Pd-catalyzed hydrogenation of 1,2-dimethylcyclopentene? 

A. trans-1,2-dimethylcyclopentane B. cis-1,2-dimethylcyclopentane C. a mixture of trans and cis-1,2-dimethylcyclopentane D. 1,1-dimethylcyclopentane

The answer is B.

can you please explain why?