A firm’s average total cost is Rs. 30 at 5 units of output and Rs. 32 at 6 units of output

A firm’s average total cost is Rs. 30 at 5 units of output and Rs. 32 at 6 units of output. The marginal cost of producing
6th units is–

1. Rs.2
2. Rs.12
3. Rs.32
4. Rs.42