WAEC BIOLOGY 2003 : Which of the following constitutes the cellular components of the mammalian blood?

Which of the following constitutes the cellular components of the mammalian blood? I. Haemoglobin Erthrocytes III. Leucocytes IV. Platelets
A. I, II and III only
B. I, III and IV only
C. II, III and IV only
D. I, II, III and IV