Which technique (cryptography or steganography) is used in each of the following cases for confidentiality? a) A student writes the answers to a test on a small piece of paper, rolls up the paper, and inserts in a ballpoint pen, and passes the pen to another student. b) To send a message, a spy replaces each character in the message with a symbol that was agreed upon in advance as a characters replacement. c) A company uses special ink on its checks to prevent forgeries. d) A graduate student uses watermarks to protect her thesis, which is posted on her website.