Choosing the right preposition

Could you please tell me which preposition would be the best choice for the gaps given in the sentences below?

1. You could tell _______ her voice that she was scared.

A. by

B. From

C. At

(I'm confused about whether to choose "by" or "from". Are both of these OK?)

2. She thinks I'll have to work _______ my pronunciation.

A. On

B. At

C. In

(I've heard "on" used in this situation, but a friend of mine says "at" is also possible. Is he right about it?)

Thank you!