Fill in the blanks in 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 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 and air traffic control.
__________
A was looked at
B was viewed
C was observed
D was witnessed