Regulatory speaking, the pilot must ensure that all terrain and obstacles along the route of flight, except takeoff and landing, are cleared vertically by no less than _________________ during the day and ________________ at night?
a) Day: 500 feet; Night: 1000 feet
b) Day: 1000 feet; Night: 2000 feet
c) Day: 2000 feet; Night: 3000 feet
d) Day: 3000 feet; Night: 4000 feet