Question
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Which of the following are not electromagnetic waves?
A. Cosmic rays
B. Gamma rays
C. β-rays
D. X-rays

Answer
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Hint: Electromagnetic radiation is a form of energy produced when charged particles move by vibrating magnetic and electric fields. An electromagnetic spectrum is the range of all electromagnetic radiations. To solve this question we check the electromagnetic spectrum, by doing that it would be easy to find the odd one from the options.

Complete step by step answer:
We know what electromagnetic radiation and electromagnetic spectrum is. Electromagnetic spectrum consists of, from shortest wavelength to longest wavelength:
Gamma rays-X rays-Ultraviolet rays-Visible light-Infrared-Microwaves-Radio waves
From this it is clear that cosmic rays are not part of the electromagnetic spectrum, and thus they are not electromagnetic radiation.
Cosmic rays are high energy outer space particles that consist mainly of protons. Or we can say that they are subatomic particles that travel at fractions of the speed of light.
Hence, the correct answer is option A and C.

Additional Information
The electromagnetic spectrum consists of electromagnetic waves. Electromagnetic waves are formed due to the vibration formed between electric and magnetic fields.
Gamma ray is formed by the radioactive decay of atomic nuclei, nuclear explosion, lightning etc. They have the smallest wavelength and the highest energy in the spectrum.
A β-ray is a high energy radiation produced by the atomic nucleus’s radioactive decay by β-decay process. It is used in the medical field for treatment of various diseases like eye, bone cancers. They are also used as tracers.
X-ray is a high energy electromagnetic radiation. Its wavelength ranges from $\text{10pm-10nm}$.X-rays are highly used in medical field mainly for checking fractures

Note:
Cosmic rays are not electromagnetic radiation since they are high energy outer space particles. But β-ray is also not really an electromagnetic radiation because they are charged particles formed during β-decay. But when comparing with cosmic ray, β-ray ray has high chances of being included as an electromagnetic radiation. And hence B-ray is concluded as an electromagnetic radiation and cosmic ray is not.