$$2 \times 3 = 6$$  $$27 - 6 = 21$$ 
A container had $27$ ℓ of longan drink. The drink is made up of three $2$-ℓ\textbf{~}bottles of longan syrup and some water. What was the volume of water used to make the drink? 

1: 6ℓ 
2: 18ℓ 
3: 21ℓ 
4: 27ℓ 