Generally students should be taught to act appropriately to the situation. Although you wouldn't normally use carb heat for more than 3 secs on the ground, if you have evidence of carb icing at the hold would you really only use 3 secs worth?
A and C
I don't like my students to apply full-throttle with carb heat hot because (so I've been told) it can cause detonation. Previously I'd been teaching studes differently.
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0 to 2000ft in 10 minutes