A rectangular sump of dimensions 6 m × 5 m × 4 m is to be built by using bricks to make the outer dimension 6.2 m × 5.2 m × 4.2 m. Approximately how many bricks of size 20 cm × 10 cm × 5 cm are required to build the sump for storing water?

1. (a)1500
2. (b)3000
3. (c)15000
4. (d)30000