You are required to draw blood from a patient and to keep it in a test tube for analysis of blood corpuscles and plasma. You are also provided with the following four types of test tubes. Which of them will you not use for the purpose?

To study blood corpuscles and plasma, the blood must be kept from clotting.
Calcium ions ($Ca^{2+}$) are essential for blood coagulation (they are needed for the conversion of prothrombin into thrombin and in other steps of the clotting cascade).
A test tube containing calcium bicarbonate would supply extra $Ca^{2+}$ ions and promote clotting, so it must not be used.
Sodium oxalate (and citrate) removes $Ca^{2+}$ by forming insoluble calcium oxalate, so it prevents clotting.
Heparin is a natural anticoagulant that prevents the conversion of prothrombin into thrombin.
Chilling slows down the enzymatic reactions of clotting, so a chilled tube also delays coagulation.
So the test tube containing calcium bicarbonate is the one you will not use.