How do you use prepositions with adjectives? (Quiz for Upper Intermediate/Advanced level)

Добрый вечер, Мария.
В правильном ответе на предложение №7 это показано:

The teacher was disappointed ___ the behavior of the class.

YOUR ANSWER: by
CORRECT ANSWER: in

Но я думаю, что мой ответ так же хорош, сравните это:

On the other hand, «disappointed by» usually indicates that somebody has done something specific to cause you to be disappointed:

I was disappointed by Fred. He said he’d give me a ride, but he never showed up!

(The «by» form is occasionally used with objects, but most of the time only makes sense for people.)

«Disappointed in» usually indicates a deeper level of disappointment with the nature of somebody or something, or repeated problems with them, and often indicates that the speaker has lost faith in someone’s ability to do what’s expected of them:

I’m very disappointed in Bill. I thought he had experience with this job, but every time he does it he does something wrong.

Я тоже думаю, что «поведение класса» — это то, что можно изменить, надеюсь)

Всего хорошего,
Арвид