___ are in group 18 a. noble gases
b. halogens
c. alkali metals
d. alkaline earth metals

Explanation:

Groups are numbered 1–18 from left to right. The elements in group 1 are known as the alkali metals; those in group 2 are the alkaline earth metals; those in 15 are the pnictogens; those in 16 are the chalcogens; those in 17 are the halogens; and those in 18 are the noble gases.

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answer by yourhope:

how do models compare to real-world objects?

d. most models represent a real-world object in a simpler way.

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