If abcd is parallelogram, AB = 2i + 4j - 5k and AD = i + 2j + 3k, then the unit vectors in the direction of BD is

a. 1/√69 (i + 2k - 8k)
b. 1/√69 (i + 2j - 8k)
c. 1/√69 (-i -2j + 8k)
d. 1/69(-i -2k + 8k)