Option a is the correct answer. Surkhi is used as a substitute for sand for concrete and mortar, and has almost the same function as of sand but it also imparts some strength and hydranlicity. Surkhi is made by grinding to powder burnt bricks, brick-bats or burnt clay ; under-burnt or over-burnt bricks should not be used, nor bricks containing high proportion of sand. According to some archaeological studies, there is evidence of the use of Surkhi in various structures built during the Kushan period. For example, in Mathura, which was one of the major centers of the Kushan empire, Surkhi was used in the construction of brick stupas, shrines, and platforms. The excavations at Mathura revealed five levels of occupation, dating from the Mauryan to the Gupta periods. The third and fifth levels belonged to the Kushan period and showed the use of Surkhi in various structures.