Which one of the following conditions is a typical characteristic of critical flow (symbols have the usual meanings)

• A. <img src="/_files/images/civil-engineering/upsc-civil-service-exam-questions/80-495-1.png">
• B. <img src="/_files/images/civil-engineering/upsc-civil-service-exam-questions/80-495-2.png">
• C. <img src="/_files/images/civil-engineering/upsc-civil-service-exam-questions/80-495-3.png">
• D. <img src="/_files/images/civil-engineering/upsc-civil-service-exam-questions/80-495-4.png">