Answer:
It's time for the class to end, but Mr. Johnson doesn't realize that.
Explanation:
The first sentence would be correct because of where the comma is placed. It separates the two clauses. However, the second sentence and third would be correct for the closing punctuation which the first lacked. The third would be reliable for the spelling of "doesn't" as in sentence one, it was spelled without and apostrophe and in sentence 2 it is spelled as "coesn t...". So none of them is correctly punctuated all the way so the sentence above, would be accurate after combining all three sentence choices to form a correctly punctuated sentence.
