B. let down
Feedback let up - 1. to become less severe 2. to make less effort.
let out - 1. to make a loud sound 2. to loosen a piece of clothing 3. to take money from somebody for a rent. 4. to set somebody free. let down - 1. to upset 2. to make longer.
let off - 1. not to punish 2. to make explosion