A perfectly competitive firm produces digital devices using capital and labour. The wage rate is $10, the rental cost of capital is $10, and the price of a digital device is $20. Suppose that the cost-minimising way to produce 20 digital devices is to use 4 units of labour and 4 units of capital, and the cost-minimising way to produce 21 digital devices is to use 4.2 units of labour and 4.2 units of capital. The marginal cost (MC) of producing the 21st digital device is:

$4

$5

$8

$3

Which of the following and why?