B. keep back
Feedback go on - to continue.
keep back - 1. not to tell something you know 2. not to show your emotions 3. not to give something you're supposed to give.
fall for - 1. to be misled/deceived 2. to love somebody.
keep up - 1. to continue 2. to go as fast as someone next to you 3. to succeed 4. to be up to date/to be aware of actual information on some subject