Chemical bonds
a. hold atoms together to form compounds.
b. are always covalent.
c. are always ionic.
d. combine compounds to form elements.

The best answer is A.

A chemical bond  is said to be a lasting attraction occurring  between atoms that causes them to form chemical compounds.

A bond that results from an attraction of atoms due to the opposite charges (electrostatic charges) on them is called an ionic bond. When two atoms share electrons , they get bonded together in a type of bond called a covalent bond. 

Covalent and ionic bonds are the strongest types of chemical bonds that occur between atoms.

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

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