Which revision correctly combines these sentences?

My computer was working very slowly. I had to restart it for the updates to take effect.

A My computer was working very slowly, I had to restart it for the updates to take effect.

B My computer was working very slowly and I had to restart it for the updates to take effect.

C My computer was working very slowly I had to restart it for the updates to take effect.

D My computer was working very slowly; I had to restart it for the updates to take effect.