Which of the following adds a subordinate clause to the main clause below?

Question

I woke up early this morning.
A.
I woke up early this morning; the birds were singing so loudly.
B.
I woke up early this morning because the birds were singing so loudly.
C.
I woke up early this morning. The birds were singing so loudly.
D.
I woke up early this morning, and the birds were singing so loudly.

Answer 1

The option that adds a subordinate clause to the main clause is:

B. I woke up early this morning because the birds were singing so loudly.

In this sentence, "because the birds were singing so loudly" serves as a subordinate clause that provides a reason for waking up early.