You are given 250.0 mL of 0.100 M CH₃CH₂COOH (propionic acid, K_a = 1.35 x 10^-5). You want to adjust its pH by adding an appropriate solution. What volume would you add of 

(a) 1.00 M HCl to lower the pH to 1.00; 

(b) 1.00 M  NaCH₃CH₂COO to raise the pH to 4.00; 

(c) Water to raise the pH by 0.15 unit?