What is the equation for the firm's marginal cost?

Select one:

a$.MC=2,000-20Q+0\mathrm{.}75Q^2$

b$.MC=7,000-20Q+0\mathrm{.}75Q^2$

c$.MC=2,000-10Q+0\mathrm{.}25Q^2$

d$.$ None of the above.