Carbon dioxide is my guess ;)
Answer:
The answer is D i think :)
Trials.
Methods, control, and variables aren’t the experiments themselves.
Prenatal medical procedure in which high frequency sound waves are directed into a pregnant woman's abdomen is called ultrasound sonography.
Ultrasound imaging or sonography is often used in medicine as the nondestructive procedure to visualize muscles and many internal organs as well as fetuses during routine and emergency prenatal care..
we know that

so

therefore
<u>The answer is</u>
to convert millimeters into kilometers
Antoine should divide millimeters by 