That's very interesting. I had never thought about it before. Let's look through all of the ten possible digits in that place, and see what we can tell:
-- 0: A number greater than 10 with a 0 in the units place is a multiple of either 5 or 10, so it's not a prime number.
-- 1: A number greater than 10 with a 1 in the units place could be a prime (11, 31 etc.) but it doesn't have to be (21, 51).
-- 2: A number greater than 10 with a 2 in the units place has 2 as a factor (it's an even number), so it's not a prime number.
-- 3: A number greater than 10 with a 3 in the units place could be a prime (13, 23 etc.) but it doesn't have to be (33, 63) .
-- 4: A number greater than 10 with a 4 in the units place is an even number, and has 2 as a factor, so it's not a prime number.
-- 5: A number greater than 10 with a 5 in the units place is a multiple of either 5 or 10, so it's not a prime number.
-- 6: A number greater than 10 with a 6 in the units place is an even number, and has 2 as a factor, so it's not a prime number.
-- 7: A number greater than 10 with a 7 in the units place could be a prime (17, 37 etc.) but it doesn't have to be (27, 57) .
-- 8: A number greater than 10 with a 8 in the units place is an even number, and has 2 as a factor, so it's not a prime number.
-- 9: A number greater than 10 with a 9 in the units place could be a prime (19, 29 etc.) but it doesn't have to be (39, 69) .
So a number greater than 10 that IS a prime number COULD have any of the digits 1, 3, 7, or 9 in its units place.
It CAN't have a 0, 2, 4, 5, 6, or 8 .
The only choice that includes all of the possibilities is 'A' .
They are perpendicular because when you change 2x+8y=16 to slope intercept form it becomes y= -1/4x+2 and the slope (-1/4) is the negative reciprocal of 4, which is the slope for your other equation.
