Change the pig's joke

From the meaning of the question: 1 cow =6 pigs, 2 pigs = 10 sheep, 3 sheep =20 chickens,

400 chickens for sheep: 400÷20×3=60 (one);

60 sheep can be replaced by pigs: 60÷ 10×2= 12 (head);

12 pig for cow: 12÷6=2 (head).

Answer: 400 chickens can be exchanged for 2 cows.

Therefore, choose: B.