im kind of confused about the concept on agency

Question

Tania had apparent authority to buy a car for Sharon, provided the price was less than $10,000. Tania has now entered into an agreement to buy a car, on Sharon's behalf, from Douglas for $15,000.

the answer on the quiz was

Sharon can purchase the car if she chooses to

why isnt the answer Sharon must purchase the car.??

i mean, tania had apparent authority to buy a car and even if tania didn't follow instructions and still bought the car wouldnt sharon still be bound by the deal, so wouldn't she had to purchase the car???

Answer 1

Already answered.

http://www.jiskha.com/display.cgi?id=1228913688