Two boats A and B started rowing downstream from P simultaneously at different constant speeds and another boat C started rowing upstream from Q at the same time. If A crossed C at a point, which is at a distance of three-fourth of PQ from P and then B crossed C at a distance of two-third of PQ from P, and the upstream speed of boat C is 1 km/h, then which of the following gives the difference between the upstream speeds, in km/h, of boats A and C?