想了一下，第三題也許是這樣？

Assuming the water body behave both as a continuum (therefore 单值连续 can apply ) and as a discreet set with each unit (fixed position in space) small enough to hold a water molecule.

Applying result from 2, there exist a position whose temperature history is identical to the temperature history of some particular molecule. Once can show this being true just by switching coordinates.

This particular molecule is the desired one.