Radon was once commonly used to treat cancer. The radiation it gives off kills cancer cells. However, the element must be used with great care because radiation can kill healthy cells as well. In fact, the bad side-effects of radiation therapy are caused by the killing of healthy cells by radiation. Today, radon is not as widely used for the treatment of cancer. Radon is a radioactive element. A radioactive element is one that gives off radiation and breaks down to form a different element. Radon is formed when heavier radioactive elements, like uranium and thorium, break down. In turn, radon breaks down to form lighter elements, such as lead and bismuth.

Correct Option: D

Radon was once commonly used to treat cancer. The radiation it gives off kills cancer cells. However, the element must be used with great care because radiation can kill healthy cells as well. In fact, the bad side-effects of radiation therapy are caused by the killing of healthy cells by radiation. Today, radon is not as widely used for the treatment of cancer. Radon is a radioactive element. A radioactive element is one that gives off radiation and breaks down to form a different element. Radon is formed when heavier radioactive elements, like uranium and thorium, break down. In turn, radon breaks down to form lighter elements, such as lead and bismuth.

In vascular plants, phloem is the living tissue that carries organic nutrients, in particular, sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark. The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis. This is called translocation.

Correct Option: A

In vascular plants, phloem is the living tissue that carries organic nutrients, in particular, sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark. The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis. This is called translocation.

Hydras are generally sedentary and sessile.

Correct Option: C

Hydras are generally sedentary and sessile.

Ichthyophis lacks tongue. It is sometimes called the Asian caecilians. It is limbless amphibian.

Correct Option: C

Ichthyophis lacks tongue. It is sometimes called the Asian caecilians. It is limbless amphibian.

Toxins are substances created by plants and animals that are poisonous to humans. Most toxins that cause problems in humans are released by microorganisms such as bacteria, viruses, fungi, rickettsiae or protozoa. For example, cholera is due to a bacterial toxin.

Correct Option: D

Toxins are substances created by plants and animals that are poisonous to humans. Most toxins that cause problems in humans are released by microorganisms such as bacteria, viruses, fungi, rickettsiae or protozoa. For example, cholera is due to a bacterial toxin.