Chemistry Paper Chromatography

Question

A chemist wishes to separate and identify a mixture of substances using paper chromatography. The diagram shows the apparatus used. The solvent is water.The solvent front is allowed to reach the top of the paper before the chemist removes the paper from the solvent. Which problem does this cause?
A This causes the spot nearest the bottom of the paper to catch up with the spot above it
B This makes it impossible to calculate the Rf values
C This makes it impossible to use a locating agent
D This results in a safety hazard caused by the solvent fumes

Answer 1

This isn't really that clear, because you will get different results (answers) if the solvent front just reached the top of the paper, but didn't continue to run versus if it reached the top and the chromatography continues for some time thereafter.  Let us assume the latter.  Then the most likely answer is (B) because the Rf is the distance the spot moves divided by the distance the solvent moved and one would have no reliable measure of how far the solvent moved.