Context: Northern pintails eat plants that grow underwater or in mud. The shape of the 's beak is adapted to filter through mud for food.
The  gathers muddy water in its beak and then pushes it out through gaps along the sides. Bits of food, such as plant roots, are left behind inside the pintail's beak.
Figure: northern pintail.
Which bird's beak is also adapted to filter through mud?
A. common nighthawk
B. black swan
From the given choices, select the correct answer and reply with the letter of the chosen option.

B