There is ONE sentence in the choices below that does NOT have a grammar error by shifting mood or verb voice incorrectly. Which sentence is it?

Question

Read the first two chapters, and then you should meet with the study group.
When the operator turned on the machine, a squeaking sound was heard.
All members should register for the trip by noon Friday, and don't be late.
Two children left the classroom without permission, but they were found immediately.

Answer 1

The sentence that does NOT have a grammar error by shifting mood or verb voice incorrectly is:

"Two children left the classroom without permission, but they were found immediately."