QUESTION 28 Select the correct statement(s) regarding orthogonal frequency division multiplexing (OFDM). o a. OFDM is a multicarrier modulation concept where user data is transmitted over several subcarriers in parallel b. OFDM subcarriers are not separated by guard channels (i.e., subcarriers overlap one another in the frequency domain) c. while OFDM signals spread user signals across a wider frequency spectrum, it is not considered a spread spectrum technique d. all of the statements are correct