As pilots get closer to the speed of sound:
O
A. there is not enough thrust to propel the airplane past the sound
waves.
O
B. the airplane becomes more difficult to fly because there is more
drag from air molecules.
O
C. the air molecules move out of the way more quickly, which creates
a sonic boom.
O
D. they notice that the airplane uses less fuel because the pull of
gravity decreases.

d: the voltage of the secondary coil is less than that of the primary coil.

im not suree sorry if wrong

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.