Persons younger than 18 years old are not eligible to vote. let a be age in years.
which inequality models this situation?
a.
a < 18 and a may be a negative number
b.
a < 18 and a must be a positive number
c.
a > 18 and a may be a negative number
d.
a > 18 and a must be a positive number

D because a has to be younger than 18 and it has to be positive because we are never a negative age. do you understand when i mean?

The answer is D because the age has to be greater than 18 (including 18 years as well.) When the age is either going to be 18 or greater, you can't go back down to negative numbers because you increase age by one each year, not minus numbers, or decimals. For example, you go 1 year, 2 years, 3 years, not 1 year, 0 years, -1 years. I hope you understand that but if not just tell me in the comments :)