The diagram shows a food web.

A flowchart. Cycle 1: 1, Grass. 2, Grasshopper. 3, Shrew or Mouse. 4, Fox. Cycle 2: 1, Grass. 2, Mouse. 3, Fox or Snake. Cycle 3: 1, Grass. 2, Rabbit. 3, Fox.

Which is the producer?

grass
grasshopper
mouse
fox