21. The simplest way to understand how a jet engine works is to watch air
A B C
escapes from a balloon.
D
A. 1. A more simple way 2. Simpler ways
3. The most simple way 4. A simpler way than
B. 1. to understand that how 2. understanding that how
3. to understand that 4. understanding that
C. 1. watching 2. is to be watching
3. to watch 4. is watched
D. 1. to escape from a balloon 2. escaping from a balloon
3. will escape out of balloons 4. escapes out of balloons