im kind of confused about the concept on agency

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???

2 answers

No, the authority was to buy for up to 10K. Sharon is not obligated in any way to purchase the car, but she can purchase it if she wants to.
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