A vector is given by r1 = 2i + 6j – 3k. Which of the following vectors are normal (perpendicular) to r1? (Select all that apply)
a. 6i - 2j- 0k b. 3i+0j + 2k c. 1i + 4j - 2k d. 2i - 3j +6k e. -3i + 6j + 2k