Under the Inland Navigation Rules, a manned vessel being towed in restricted visibility shall sound which fog signal at intervals of not more than 2 minutes?

Why C is correct

33 CFR §83.35(e) specifies that a vessel towed, or the last vessel of a tow if more than one vessel is towed, shall sound four blasts — one prolonged followed by three short blasts — at intervals of not more than 2 minutes. One prolonged followed by two short blasts is the signal for vessels such as those not under command or restricted in ability to maneuver per §83.35(c).