【Question 18】
Xiao Dong is not both proficient in English and French. If the above statement is true, then the following statement that is true is $\qquad$.(Fill in $A, B, C, D$, E)
A. Xiao Dong understands English but not French.
B. Xiao Dong understands French but not English.
C. Xiao Dong neither understands English nor French.
D. If Xiao Dong understands English, he definitely does not understand French.
E. If Xiao Dong does not understand French, then he must understand English.
【Analysis and Solution】
Logical reasoning.
Xiaodong is not both proficient in English and French.
There are 3 cases:
(1) Xiaodong understands English but not French.
(2) Xiaodong understands French but not English.
(3) Xiaodong neither understands English nor French. Therefore, the answer is $D$.