Which sentence is a complex sentence?

a. we are out of toothpaste.
b. i brushed my teeth, but mom had no toothpaste.
c. i left the store before i remembered it.
d. i will buy more toothpaste tomorrow.

which sentence is a complex sentence?

a. the lockers in my old school were small.
b. most of them were in the hallway.
c. i hated mine because it was outside the gym.
d. the gym is four floors away from my home room.