11. D.

Let the number of people who participate in both mathematics and foreign language courses be $x$. By the principle of inclusion-exclusion, we have
$$
70+54-x=93 \Rightarrow x=31 \text {. }
$$

Therefore, the number of people who only participate in the mathematics course is
$$
70-31=39 \text {. }
$$
11. A school's eighth grade has 93 students, each participating in math or foreign language courses, or both. It is known that 70 people participate in the math course, and 54 people participate in the foreign language course. Then the number of students who only participate in the math course and not the foreign language course is ( ) people.
(A) 16
(B) 23
(C) 31
(D) 39
(E) 70