Emma has a bird feeder on her balcony. A bird ate at the feeder and then flew away. The bird's vertical height is shown in the graph below.

In the graph, t represents the time, in seconds and f(t) represents the vertical height, in feet, of the bird from ground level. Emma's balcony is 21 feet above the ground. What is the lowest height of the bird and when does it occur?

The lowest height of the bird is 21 feet and occurs after 1 second.

The lowest height of the bird is 12 feet and occurs after 3 seconds.

The lowest height of the bird is 9 feet and occurs after 6 seconds.

The lowest height of the bird is 3 feet and occurs after 12 seconds.