expr命令可以对字符串进行操作，已知某个命令执行后结果是"a book"，可以是以下（ ）命令。
选项：

A:expr substr "This is a book" 9 6;
B:expr substr "This is a book" 8 6;
C:cut "This is a book" -d' ' -f3,4;
D:echo "This is a book" | cut -d' ' -f3,4;
E:echo "This is a book" | awk '$NR=1{print $3,$4} ';
F:echo "This is a book" | awk $NR=1{print $3,$4} ; G:echo "This is a book" | awk '{print $3,$4} '; H:echo "This is a book" | awk '$NR==3{print $1,$2}'; I:echo -e "This is a book" | awk 'NR==3{print $1,$2}'; J:echo -e "This is a book" | awk 'NR==3{print}'