Of the following statements, which one is true?

Select one:

a$.\text{'}$A$\text{'}$ items usually account for about $70\%-80\%$ of the total usage value.

b$.$ About $50\%$ of the items usually account for $50\%$ of the value.

c$.\text{'}$C$\text{'}$ items should be given the top priority in inventory management.