Question $17(Mandatory)(1$ point$)$

Which $of$ the following functions $is$ always increasing?

$af(x)=(-9\mathrm{.}1{)}^{-x}$

$bf(x)=(9\mathrm{.}1{)}^{-x}+1$

$cf(x)=(9\mathrm{.}1{)}^x$

$df(x)=(9\mathrm{.}1{)}^{-x}$