help me!~中五數列一問。

Since sqrt b = ac, so b^2 = ac, or b/a = c/b, that means a, b and c belongs to a G.S.
(a) since b/a = c/b, (b/a)^3 = (c/b)^3, b^3/a^3 = c^3/b^3, that means a^3, b^3 and c^3 is also a G.S.
(b) since b^2 = ac
log(b^2) = log(ac)
2 log b = log a + log c
so log b - log a = log c - log b,
therefore, log a, log b and log c is an A.S.
(c)
4^b/4^a =4^(b -a)
4^c/4^b = 4^(c - b).
since b - a is not equal to c - b, therefore, 4^a, 4^b and 4^c is not a G.S.
Thats why answer is A.