$f(p,i)=99p-0\mathrm{.}8-0\mathrm{.}0025p^2$

$(a)$ Find the weekly sales $if$ the average list price $isp=$$$19,100$ and the manufacturer $is$ offering $a4\%i=4f_p(p,i)$ and $f_i(p,i).$

$f_p(p,i)=$

What does $f_p(p,i)$ represent?

$A.f_p(p,i)$ gives the rate $at$ which weekly sales are changing per unit change when neither price nor interest rate remains constant.

$B.f_p(p,i)$ gives the rate $at$ which weekly sales are changing per unit change when price and interest rate remain constant.

$C.f_p(p,i)$ gives the rate $at$ which weekly sales are changing per unit change $in$ price when the interest rate remains constant.

$D.f_p(p,i)$ gives the rate $at$ which weekly sales are changing per unit change $in$ interest when the price remains constant.

$f_i(p,i)=$

What does $f_i(p,i)$ represent?

$A.f_i(p,i)$ gives the rate $at$ which weekly sales are changing per unit change when price and interest rate remain constant.

$B.f_i(p,i)$ gives the rate $at$ which weekly sales are changing per unit change $in$ interest when the price remains constant.

$C.f_i(p,i)$ gives the rate $at$ which weekly sales are changing per unit change when neither price nor interest rate remains constant.

$D.f_i(p,i)$ gives the rate $at$ which weekly sales are changing per unit change $in$ price when the interest rate remains constant.

$(c)$ What would $be$ the approximate effect $on$ weekly sales $if$ the price $is$ $$19,100$ and interest rates rise from $4\%to5\%?$

The effect would $be$ a weekly sales decrease $of$ $

$.$