How do you solve this science question

**Lesson Steps**:

You take 7-3 such is 4 and you know that the smaller number was -with a bigger number so the must be - something and you then take -+4and the answer will be -4

**Answer**:

Ball A will hit ball c and ball B will hit ball D

**Grade**: 3

**Level**: Primary

**Subject**: Science

**Topic**: Kinetic energy