If triangle $RST$ is an acute triangle, then the measure of angle $S$ must be

• A. $<90°$
• B. $>90°$
• C. $=90°$
• D. $<180°$

Answer: (A)

Verified step by step guidance

1. Recall the definition of an acute triangle: a triangle where all three interior angles are less than 90 degrees.
2. Since triangle RST is acute, each angle, including angle S, must satisfy the inequality \(\angle S < 90^\circ\).
3. Understand that if any angle were equal to or greater than 90 degrees, the triangle would be right or obtuse, not acute.
4. Therefore, angle S cannot be equal to 90 degrees or greater than 90 degrees in an acute triangle.
5. Conclude that the measure of angle S must be less than 90 degrees, i.e., \(\angle S < 90^\circ\).