At a constant temperature, an ideal gas is compressed from $6\mathrm{.}0$ liters to $4\mathrm{.}0$ liters by a constant external pressure of $5.$ atm. How much work is done on the gas?

$($a$)w=+10$ liter atm

$($b$)w=-10$ liter atm

$($c$)w=+30$ liter atm

$($d$)w=-30$ liter atm

$($e$)$ The answer cannot be calculated.