When the snail walks in OA section, the function of x about t increases linearly, and reaches the maximum when it reaches point A. When it walks in AB section, the distance from point O is equal, which is the radius of a fan. When it walks in the BO section, the function of x about t decreases linearly, and decreases to 0 when it reaches the o point.

4 1、A

Step 1: Insert the first program into the four spaces formed by the original three programs. There are four insertion methods.

Part II: Insert the second program into the five spaces formed by the four programs in the first step. There are five insertion methods.

Therefore, a * * * has 4×5=20 possibilities.