1 .
How many strings can be obtained by rearranging the letters of the word
POOPERSCOOPER
(a)
${13 \choose 4}{13 \choose 3}{13 \choose 2}{13 \choose 2}{13 \choose 1}$
(b)
$13!$
(c)
${13 \choose 4}{9 \choose 3}{6 \choose 2}{4 \choose 2}{2 \choose 1}$
(d)
${13 \choose 4}{9 \choose 3}{6 \choose 2}{4 \choose 2}$