Which of These Statements About White Blood Cells (WBCs)is TRUE

Which of these statements about white blood cells (WBCs) is TRUE?

A) They are responsible for the exchange of O2 and CO2.
B) They are active in the clotting mechanism of the body.
C) They are produced by the liver according to the demand of the body.
D) They are responsible for fighting infection and assisting with immunity.