Awill masquerade as a seemingly useful program while actually compromising system security and possibly acting as a "back door," allowing additional hack tools and access to the system.

Trojan

Explanation:

According to my research on different computer viruses, I can say that based on the information provided within the question the type of computer virus being defined is called a Trojan. This is a virus that (like defined in the question) acts as a "back-door" into the victim system. This was named after the Trojan Horse strategy used by the Greeks in order to get into the city of Troy.

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a. the moon was locked into orbit by the earths gravity as it was passing by and capture hypothesis i think