Which of the following sentences is correctly punctuated?

A I am tired because I have been working since 7:00 am this morning.

B I am tired because I have been working since 7:00 am this morning.

C I am tired because I have been working since 7:00 am, this morning.

D I am tired because I have been working since 7:00 am, today.

Correct Answer: Option A

The comma after the word "morning" is unnecessary because the sentence is already complete. The other options are grammatically incorrect.