Answer:
C.
Explanation:
Answer A is incorrect because a compound doesn't have to be made with only metals.
Answer B is incorrect because that is actually the definition of a pure element.
Answer C is correct because to make a compound, there has to be a chemical reaction where bonds are formed and an entirely new substance is created.
Answer D is incorrect because compounds cannot be separated by physical means.
Hope this helps :D
The answer is B that is sound wave. Sound wave required a medium to travel.
Mechanical wave is not capable of transfer its energy through vacuum . It required a medium to travel and sound wave is an example of mechanical wave that required a medium to travel.
So, All other wave that are listed here doesn't required a medium to travel, among all only sound wave requires a medium to travel.
