If 7th and 13th terms of an A.P. be 34 and 64 respectively, then its 18th term is

(c) 89

$a_{7}=34$

$\Rightarrow a+6 d=34$       ....(1)

Also, $a_{13}=64$

$\Rightarrow a+12 d=64$       ....(2)

Solving equations $(1)$ and $(2)$, we get:

a = 4 and d = 5

$\therefore a_{18}=a+17 d$

$=4+17(5)$

$=89$