Fill in the blanks of the passage
from the given options:
First Trials on Airplanes
Another significant phase in airplane travel
_____ a few years later. Extensive research and
development went into designing wartime planes
that were used ____ the First World War. These
planes were much faster, reaching speeds of 125
mph and flew to the heights of 15,000 feet.
____ airplanes owe their development to the
First World War, they are significantly popular today
as passenger carriers. A flight is ___?___ today than
earlier, with greater facilities ____ air traffic
control.
A safer
B more safer
C safest
D safe