Assertion: The matrix used in gel electrophoresis should have controllable pore size.
Reason: Agarose concentration can be changed to change pore sizes.

Both Assertion (A) and Reason (R) are correct but R is not the correct explanation of A.

Agarose is a polysccharide obtained from red algae. Agarose dissolves in hot water and when it is cooled, it forms gel. Pore size depends upon agarose concentration. In general, a 1% (w/v) gel will have a pore size of 150 nm. While a 0.16% gel has pore size of 500 nm.