If we find that nominal (current dollar) GDP fell last quarter, then we can conclude which of the following is true:

If inflation was positive then real GDP must, in all cases, have risen

If we had falling prices, or deflation, it will always exactly offset the fall in nominal, resulting in zero growth for real GDP in all cases

If inflation was positive then real GDP must have fallen as well

If inflation was zero, then real GDP growth rate will also be zero