Surreptitiously, as described by Merriam-Webster, is "done, made, or acquired by stealth." Essentially, it's doing something sneakily or in a stealthy way.
Not c , not d , a or b <3
The tone of the letter was too informal and lacks details needed for good resolution. This is the answer because the paragraph was opinionated, and DID NOT state specifics.
You can use it to detect things that are unusual, which we already have like blood flow, the amount of beats your heart gives off, to track distances from your home to another place, to trace people's location, and so on.
