Compute the value of a + b + c + d. Choose the most appropriate option:	A) 40	B) 50	C) 60	D) 70	E) 80
Let s = a + b + c + d. Subtracting s from each side of the equations gives us   ab - a - b = 13-s   bc - b - c = 27- s   cd - c -d = 30 - s   da - d - a = 17 - s   Adding one to each side of the equations and applying Simon's Favorite Factoring Trick gives   us   (a - 1)(b - 1) = 14 - s   (b - 1)(c - 1) = 28 -s   (c - 1)(d - 1) = 31 - s   (d - 1)(a - 1) = 18 - s   Multiplying the first and third equations and the second and fourth equations allows us to write   the following equality:   (a - 1)(b - 1)(c - 1)(d - 1) = (14 - s)(31 - s) = (28 - s)(18 - s)   Expanding and solving for s gives us s = 70 .   correct answer 70
<result>D</result>
Compute the value of a + b + c + d.