$(5$ pts $)17.$ In an isothermal $($constant temperature$)$ process of an ideal gas expands and does $400$ J of work. During this process what is the magnitude of the heat flow and does heat flow into the gas or out of the gas?

$($a$)$ zero

$($b$)400$ J out of the gas

$($c$)400$ J into the gas

$($d$)200$ J out of the gas

$($e$)200$ J into the gas

$($f$)$ none these answers

Please help me understand why it is going into the gas. I will like. Thanks