Stationary waves — practice question

For a stationary wave to be formed, two waves have to move through the same region in opposite directions. Which statement about the properties of these two waves must also be true?

• AThe waves must have equal frequencies, but different speeds and wavelengths.
• BThe waves must have equal speeds, but different wavelengths and frequencies.
• CThe waves must have equal speeds, frequencies and wavelengths.
• DThe waves must have equal wavelengths, but different speeds and frequencies.