2. (10 points) Vasya has 9 different books by Arkady and Boris Strugatsky, each containing one work by the authors. Vasya wants to arrange these books on a shelf so that: a) the novels "The Beetle in the Anthill" and "Waves Extinguish the Wind" (in any order) are next to each other; b) the novellas "Anxiety" and "A Tale of Friendship and Enmity" (in any order) are next to each other. In how many ways can Vasya do this?
a) $4 \cdot 7!$;
b) 9 !;
c) $9!/ 4!$
d) $4!\cdot 7!$;
e) another answer.