$16 \cdot 23$ In the angles formed by the intersection of two lines,
(A) there must be an obtuse angle.
(B) there must be an acute angle.
(C) there must be one that is not an obtuse angle.
(D) there must be two acute angles.
(2nd "Hope Cup" National Mathematics Invitational Competition, 1991)
[Solution] Considering that when two lines are perpendicular to each other, the four angles formed are all right angles, then (A), (B), (D) are not true.
Therefore, the answer is (C).