1 .
How many strings can be obtained by rearranging the letters of the word
POOPERSCOOPER
(a)
${13 \choose 4}{9 \choose 3}{6 \choose 2}{4 \choose 2}$
(b)
$13!$
(c)
${13 \choose 1}{12 \choose 4}{8 \choose 2}{6 \choose 1}{5 \choose 2}{3 \choose 3}$
(d)
${13 \choose 4}{9 \choose 2}{7 \choose 2}{5 \choose 3}$