Which of the sentences contain both an independent and a dependent clause?

a. he was listening intently to the radio, and he didn't hear the doorbell.

b. as she entered the auditorium, she tripped over a backpack that had been carelessly left in the doorway.

c. visiting the aquarium turned out to be quite an adventure; i enjoyed it thoroughly.

d. we understand the environmental concerns of the farming community, but we are aware that business owners have another point of view.