Answer:
see below
Step-by-step explanation:
The ratio of terms that are two terms apart (s4 and s6) is the square of the common ratio:
s6/s4 = r^2
r = √(8/18)
r = 2/3 . . . . . matches choices A and C
__
Using the formula for the general term, we now know enough to find the first term:
sn = s1·r^(n-1)
s4 = s1·(2/3)^(4-1)
Using s4 = 18 and multiplying by (2/3)^-3, we get ...
18·(2/3)^-3 = s1 = 18·27/8
s1 = 243/4 . . . . . matches choice A
