As per our yesterday's Verbal Expert chat, answer must be D.
"highly sensitive nature of their work" is modifying why staff was instructed. So, it is modifying the action. Due to cannot be used to modify an action.
Hence, it MUST be "because of".
"highly sensitive nature of their work" is modifying why staff was instructed. So, it is modifying the action. Due to cannot be used to modify an action.
Hence, it MUST be "because of".
![FLO – Therapy at the Club – Pre-Single [iTunes Plus M4A]](http://is1-ssl.mzstatic.com/image/thumb/Music221/v4/a4/11/88/a41188ed-0f2e-e8d5-1163-d59e373cc58c/26UMGIM30046.rgb.jpg/208x208bb.webp)
