2021 balls are arranged in a row and are numbered from 1 to 2021. Each ball is coloured in one of four
colours: green, red, yellow or blue. Among any five consecutive balls there is exactly one red, one yellow and one blue ball. After any red ball the next ball is yellow. The balls numbered 2, 20 and 202 are green. What colour is the ball numbered 2021?
(A) green
(B) red
(C) yellow
(D) blue
(E) not determined