Answer:
A.) Because we are leaving early, we must check with our teachers about missing homework.
Step-by-step explanation:
Believe it or not, you can start a sentence with "because" it just has to have a cause and effect.
Example:
Because it was raining outside, I took my umbrella.
You also have to add a coma after the "cause" of the sentence, as shown above.
Since the first answer choice meets both of these requirements, it is correct.