Solution: The statement says that two blocks are present on a lid of a container with volume of 9 m³. The mass of lid is equal to the mass of two blocks. It means that initially there are four blocks (or four atm pressure) upon 9 m³ volume.
After that four more blocks are placed on the lid. Means the pressure is increased from 4 atm to 8 atm (2 atm of lid, 2 atm of old blocks, 4 atm of new four blocks).
A mixture is a substance that is made up of 2 or more substances that are not chemically combined together, which means they can be separated without going through chemical processes, like heating or electrolysis. They can be separated just by simple physical methods such as filtration.
Since a can of peas and carrots is composed of pea and carrots, which both of them are not chemically combined, and can be separated without being heated or applying electricity, therefore, it is a mixture.