Subject: mathqa
Question: it will take 16 days for mary to complete a certain task alone . she worked for 8 days before she was joined by her sister . both of them completed the remaining task in 2 and half days . if her sister had joined her when she started the task , how many days would it have taken ?
Option: 
A: 2
B: 6
C: 5
D: 4
Answer: C