मानक प्रतीकों का उपयोग करते हुए, निम्नलिखित में से कौन सा समीकरण एक खुले चैनल (open channel) में क्रांतिक प्रवाह (critical flow) की स्थिति को सही ढंग से परिभाषित करता है?

For subcritical flow in an open channel, the control section for a gradually varied flow (GVF) profile is located:

Depth of the gradually varied flow in an open channel is determined by the method called:

Which of the following equations signifies that 'the free surface of water is parallel to the bed of the channel'? (where dh/dx represents the variation of water depth along the length of the channel)