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John.Heddles replied the topic: zero fuel weight ballast
For some larger aircraft, depending on how the weight and balance limits are set up, centre tank fuel could get tangled up with ZFW. However, at this level of study, fuel as ballast is independent of ZFW.
The other consideration is safety management. As a general idea, it is not good practice to carry fuel as ballast on light aircraft, although it is often done on heavies. Consideration is that, should you get into a situation where you end up going into the ballast fuel, you may/will end up outside the envelope.
However, I guess for the exams it is another thing for the examiner to use to get you to think about things.
Engineering specialist in aircraft performance and weight control.