My sister always wakes up early. She has to be at school by 7:00.

Which is the best way to combine these sentences?

My sister always wakes up early; she has to be at school by 7:00.
My sister always wakes up early; because she has to be at school by 7:00.

I think its A

Yes, A.

The best way to combine these sentences is:

"My sister always wakes up early because she has to be at school by 7:00."