Which of the following sentences is a compound-complex sentence?

a
Because Shane wanted a birdhouse, his father showed him the correct way to build a small, sturdy one.

b
Bradley bought a new polka-dotted umbrella, even though the one he had was pretty big and worked well.

c
The senate had been in session for two weeks when they decided to take yet another three-day break.

d
After the sharp rise in tuition costs, the students who attended college protested, and they walked out of class.