When driving a straight truck on a two-lane highway, how many feet should you stay away from the vehicle ahead?

Answer: D — 20 feet

As a general rule of thumb, it is recommended to maintain a distance of 20 feet between your straight truck and the vehicle ahead while driving on a two-lane highway. This allows for enough space in case of sudden braking or other unexpected situations. Option A (5 feet) is too close and does not leave enough room for safe maneuvering or braking. Option B (10 feet) and Option C (15 feet) are also too close and may not allow for enough reaction time in the event of an emergency. It is important to maintain a larger distance on two-lane highways as there is no median or additional lanes for vehicles to swerve to in case of an accident. Therefore, Option D (20 feet) is the safest and most recommended distance to maintain while driving a straight truck on a two-lane highway.