TIMO - 2017 - Grade 7 - Preliminary Round - Paper 1 Question 21

Question 21

A test contains 10 questions, each with four different answers, exactly one of which is correct. What is the probability that a completely unprepared student has exactly 9 correct answers?

$\frac{9}{4^{10}}$
$\frac{10\times3^{10}}{4^{10}}$
$\frac{30}{4^{10}}$
$\frac{9\times3^{10}}{4^{10}}$