Tabby is training for a triathlon. She swims at a speed of 1 mile per hour. She runs at a speed of 9 miles per hour. She wants to figure out her average speed for these two events. What is the correct answer for her?
Options:
A) 8 mph
B) 5.25 mph
C) 3.5 mph
D) 5 mph
E) 0.5 mph

Rationale: "( 1 mph + 9 mph ) / 2 = 5 mph correct option is : d"
Answer: D