Question

Consider these sentences: Jane spent all night working on the assignment. This is why she was sleepy in class.

How would you rewrite them to make the message clearer?

Jane spent all night working on the assignment, and this is why she was sleepy in class.
Jane spent all night working on the assignment. That is why she was sleepy in class.
Since Jane was sleepy in class, she spent all night working on the assignment.
Since Jane spent all night working on the assignment, she was sleepy in class.

Answer 1

Since Jane spent all night working on the assignment, she was sleepy in class.

Step-by-step explanation:

Emphasis are mostly started by a subordinating conjunction. Since is the best example of the subordinating conjunction at the moment, so it makes the sentence cleared for a person to understand why Jane was sleeping in class.