Verb + Preposition

It's so noisy that I can't concentrate (a)   my homework.

Don't worry - I'll pay (a)   the tickets.

I borrowed a pen (a)   my classmate.

I've been waiting (a)   the bus for more than twenty minutes!

Julie: "What time shall we eat dinner?" Gill: "It depends (a)   John - we'll eat when he gets home".

When we arrived (a)   the cinema, the film had already started.

Please explain this problem (a)   us.

She was listening (a)   the radio when the doorbell rang.

John worries (a)   his exam results all the time.

My flatmate listens (a)   a lot of jazz.