18.11 Given a quadrilateral $A B C D$ with a point $E$ inside it. Connecting $A E, B E, C E, D E$ divides the quadrilateral $A B C D$ into four triangles of equal area. Then, among the statements:
甲: $A B C D$ is a convex quadrilateral,
乙: $E$ is the midpoint of diagonal $A C$ or diagonal $B D$,
丙: $A B C D$ is a parallelogram,

which of the following is true?
(A) Only 甲 is correct.
(B) Only 乙 is correct.
(C) 甲, 乙, and 丙 are all correct.
(D) 甲, 乙, and 丙 are all incorrect.
(China Junior High School Mathematics League, 1987)