Is it possible to have a project with no critical activities? I think so because the total float does not have to be 0 for any of the activities. We know that total float is: \text{Total Float} = TF(i,j) = LT(j)-ET(i)-t_{ij} . In other words its true that:

LT(i,j) - ET(i,j) \neq t_{ij} for any activity right?