Tabby is training for a triathlon. She swims at a speed of 1 mile per hour. She runs at a speed of 7 miles per hour. She wants to figure out her average speed for these two events. What is the correct answer for her?  Select the correct option:
A) 8 mph	B) 5.25 mph	C) 3.5 mph	D) 4 mph	E) 0.5 mph

Rationale: "( 1 mph + 7 mph ) / 2 = 4 mph correct option is : d"
Answer: D