High and steady rate without pauses is what they wanted. The highest rate of responses generated amongst the schedules is fix ratio. It also ensures a steady rate of response because the rats know exactly how many responses will get them a food so they keep pressing steadily again and again
Variable on the other hand is more resistant to extinction
If you argue that because the rat knows the amount of times he'll need to press the lever to be reinforced means they'd either get bored or not do it until they want it, then you can also argue that because variable is random, some rats might pause for a while because they mightn't get reinforced for a long time
That's at least what the teacher told me, she could be wrong. I myself thought it was variable ratio
Edit: Just checked on wiki, all fixed schedules provide pauses after each reinforcement, so variable ratio has to be it