The marginal cost curve is generally because diminishing marginal returns implies that additional units are

The marginal cost curve is generally upward-sloping, because diminishing marginal returns implies that additional units are more costly to produce.
The graphical relation or connection which formed a U-shaped curve that is between marginal cost and yield is known as marginal cost curve and it is comparatively high when there is output in small quantities, and when the mount of product increases it declines and approaches a minimum value and then rises again.

Bc equals mb that should be your answer