For an effective discussion in class VII on the topic, "Why smoke rises up?" a science teacher should:

A. discourage questions during discussion.

B. provide a common platform to the learners.

C. acknowledge learners' responses to sustain their interest.

D. summarise and assess the learner's ideas at the end of the discussion.

In Science class, class discussions are important to learning in all disciplines because it helps students process information rather than simply receive it. In effective classroom discussions, we ask students to offer their ideas, waiting for students to think must trump the rush to finish. 

For an effective discussion in class VII on the topic, "Why smoke rises up?" a science teacher should:

• Focuses attention on student-centred learning where the teacher encourages the to indulge in self-learning techniques.
• Encourage students to ask questions and put their points of view.
• Provide a common platform to all the learners.
• Allow all the learners to participate in the discussion.
• Promote active learning by association and mutual co-operation of a group of peers to gain quality experiences.
• Acknowledge learners' responses to sustain their interest.
• Employs minimum effort and resources on teaching and motivates the learner to actively construct knowledge.
• Summarise and assess the learner's ideas at the end of the discussion.

Hence, from the above-mentioned points, it becomes clear that discouraging questions during the discussion is not the criterion for effective discussion in class.