A regression model between sales $($y in $$1,000),$ unit price $($x$1$ in dollars$)$ and television advertisement $($X$2$ in dollars$)$ resulted in the following function: $($This is a Thought $-$ critical thinking$)$ analytical type problem$).$ Try your best!

yhat $=12-4$x$1+10$x$2$

The coefficient of the unit price indicates that if the unit price is$..............$

a$.$

increased by $$1($holding advertising constant$),$ sales are expected to increase by $$40$

b$.$

increased by $$1($holding advertising constant$),$ sales are expected to decrease by $$4,000$

c$.$

decreased by $$1($holding advertising constant$),$ sales are expected to decrease by $$4$

d$.$

increased by $$1($holding advertising constant$),$ sales are expected to increase by $$400$