BECAUSE OR BECAUSE OF?

BECAUSE is followed by VERBS
He failed the test because he didn't study enough.
She wasn't allowed to apply for the job because she hasn't graduated yet.

and

BECAUSE OF is followed by NOUNS
They lost the lawsuit because of the lack of evidence.
She couldn't stop laughing because of all the jokes they were telling.

Practice:

1. I came back .......... I'd forgotten my briefcase.
a) because
b) because of
c) so that
d) to

2. I cannot come .......... the weather
a) because
b) because of
c) so that
d) to

3. I'm asking you .......... I want your opinion.
a) because
b) because of
c) so that
d) to

4. The flight was canceled .......... the snow.
a) because
b) because of
c) so that
d) to