Alice and Bob drive at constant speeds toward each other on a highway. Alice drives at a constant speed of 30 km per hour. At a certain time they pass by each other, and then keep driving away from each other, maintaining their constant speeds. If Alice is 100 km away from Bob at 7 am, and also 100 km away from Bob at 11 am, then how fast is Bob driving (in kilometers per hour)? Choose the correct option
A) 20
B) 24
C) 28
D) 32
E) 36

Rationale: alice and bob complete 200 km / 4 hours = 50 km / hour bob ' s speed is 50 - 30 = 20 km / hour the answer is a .
Answer: A