What does a decrease in a business's stock days potentially indicate?

I. it is paying it's suppliers more slowly to converse cash

II. its suppliers are unable to exert effective bargaining power

III. it exceeded its targeted sales levels

IV. it switched from standard cost to last-in/first-out inventory (stock) valuation

(which one)

a. I,II and IV only

b. III and IV only

c. II only

d. I only