18. Petra has three different dictionaries and two different novels on a shelf. How many ways are there to arrange the books if she wants to keep the dictionaries together and the novels together?
A 12
B 24
C 30
D 60
E 120
18. B Petra can arrange the dictionaries in six ways (3 choices for the first dictionary, 2 choices for the second dictionary, 1 choice for the third, giving $3 \times 2 \times 1=6$ ways). The novels can be arranged in two ways. Since the novels could be on the left of the dictionaries, or on the right, we have a total of $6 \times 2 \times 2=24$ ways to arrange the books.