3 moles of a diatomic gas undergoes an adiabatic expansion. If the internal energy of the gas decreases by 200J what is the work done by the gas.

The ans choices

-200

66.7

-66.7

+200J- this is the answer

i chose A= -200 joules because i figured sine it is adiabatic no heat is lost so the equation above is U= -W and therefore the workdone by the gas is negative. Unfortunately this is not so according to the answer can someone pls explain..thank you!