Need to make 500 mL of 20 mM CaCl2. Explain how to prepare desired solution

[franny]

from the folllowing stocks? a. 1M CaCl2
b. Calcium Chloride Hexahydrate Solid
c. 1000 ppm CaCl2
d. 10% (w/w) CaCl2

HELP!!! thankyou!