单选题 Allie lives 3 km west of Billy's house. Charlie lives 4 km north of Allie's house and 3 km west of Danny's house. What is the distance from Allie's house to Danny's house?

(A) 4 km
(B) 5 km
(C) 8 km
(D) 10 km
(E) 12 km

【正确答案】 B

【答案解析】To solve this problem, draw a diagram such as this:
[*]
Recognize that Danny's house will also be 4 km north of Billy's house, creating a right triangle. The distance from Allie's house to Danny's house is the hypotenuse, so use the Pythagorean theorem to solve:
3²+4²=c²
9+16=c²
25=c²
5=c