The correct answer is A: Signals not using the spread spectrum algorithm are suppressed in the receiver. Received spread spectrum signals are resistant to interference because signals not using the spread spectrum algorithm are suppressed in the receiver. The receiver only processes signals that match the spreading code. In spread spectrum systems, the receiver uses the same spreading code as the transmitter to despread the signal. Signals that don't have the correct spreading code appear as noise and are suppressed. This provides processing gain - the desired signal is enhanced while interfering signals (that don't match the code) are rejected. This is why spread spectrum is resistant to interference - the receiver essentially ignores signals that don't use the correct spreading algorithm. This is a key advantage of spread spectrum communications.