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It’s time for the class to end, but Mr.Johnson doesn’t realize... (the rest of the sentence was cut off)

Explanation:

The semicolon should only be used when there are two different subjects. In this case, there is only the fact that class is supposed to end, but Mr. Johnson doesn’t realize something.

There are always some cases where you dont even need a comma, but it doesn’t really fit right. If you were to say that sentence out loud, it would commemorate to use a comma, by natural spacing.

martians or other types of aliens represent a unique fear as opposed to zombies. vampires and other types of monsters.   in most sci-fi stories, aliens come from other worlds and have different or more advanced technology.   abductions and experimentation on human bodies that involve these technologies, or the use of these technologies against human beings and the use of unknown "powers", are a type of fear that's different from that of other types of monsters.