6. Ed and Ann have lunch together, Ed and Ann order medium and large lemonade drinks, respectively. It is known that the large size has a capacity 50% more than the medium size. After both have drunk $\frac{3}{4}$ of their respective drinks, Ann gives the remaining $\frac{1}{3}$ plus 2 ounces to Ed. After lunch, they find that they have drunk the same amount of lemonade. Then the total amount of lemonade they drank is ( ) ounces.
(A) 30
(B) 32
(C) 36
(D) 40
(E) 50
6. D.

Let the size of the medium lemon drink be $x$ ounces. According to the problem, we have
$$
x+\frac{3}{2} \times \frac{1}{4} \times \frac{1}{3} x+2=\frac{3}{2} x-\frac{3}{2} \times \frac{1}{4} \times \frac{1}{3} x-2 \text {. }
$$

Solving for $x$, we get $x=16$.
Therefore, the total amount they drank is $16+\frac{3}{2} \times 16=40$ ounces.