The distance between the parallel planes 4x – 2y + 4z + 9 = 0 and 8x – 4y + 8z + 21 = 0 is

• A. \(\frac{1}{4}\)
• B. \(\frac{1}{2}\)
• C. \(\frac{3}{2}\)
• D. \(\frac{7}{4}\)