Which sentence uses the underlined preposition correctly?

a.
he saw the squirrel slip off of the branch.

b.
he saw the squirrel slip off the branch.

How can you avoid the problem of groupthink? a. by choosing a group leader to direct the group b. by composing the team of only like-minded individuals c. by encouraging all members to voice their opinions d. by keeping quiet when you disagree with another group member's opinion