How many ways are there to select 3 letters from the four letters $a , b , c , d$ with repetition allowed and arrange them in a row? [2 points] (1) 56 (2) 60 (3) 64 (4) 68 (5) 72
How many ways are there to select 3 letters from the four letters $a , b , c , d$ with repetition allowed and arrange them in a row? [2 points]\\
(1) 56\\
(2) 60\\
(3) 64\\
(4) 68\\
(5) 72