According to first law of thermodynamics, when heat flows into a system and no work is done the internal energy of the system must

Explanation

The first law states that the change in internal energy equals the heat added plus the work done on the system, so with no work term every joule of heat entering raises the internal energy and the temperature rises. This is the constant volume case, which is why the molar specific heat at constant volume is smaller than at constant pressure. If the gas were allowed to expand, part of the heat would be spent doing work instead.