The number of positive integer solutions for the equation x +y +z + t = 21 is
Choose the correct choice.
A) 1140	B) 1145	C) 1240	D) 1340	E) 15640

Rationale: "the number of positive integer solutions for the equatio fx 1 + x 2 + ⋯ + xn = k ( k - 1 ) c ( n - 1 ) - where k is the number and n is number of variable in the equation . 21 - 1 c 4 - 1 = 20 c 3 = 1140 answer : a"
Answer: A