How can it become cars? It should be car isn't it?
If you want to say "car" without s then it needs "a" or "an" particle. "Cars" used for general meaning.
So it is either,
"They will use an old car", or
"They will use old cars",
Both of them are correct, and accepted by duolingo
Cars = mobil-mobil
I lost a heart :(
I agree, should be car
Pakai the salah
Kurang 'an' doang salah... pdhl di soal tdk ad kata 'sebuah'..ckckck