Correct way to express in English that a string is encoded ... using UTF-8 ... with UTF-8 ... in UTF-8?

[Costello, Roger L. via Unicode]

Hello Unicode experts!

Which is correct:

(a) The input file contains a string. The string is encoded using UTF-8.

(b) The input file contains a string. The string is encoded with UTF-8.

(c) The input file contains a string. The string is encoded in UTF-8.

(d) Something else (what?)

/Roger