In the back half of my second year as a teacher, I have learned an important principle of quizzes and tests:

Something like nine short, easy, objective questions; one long, hard, detailed, partially subjective question. (Or, if you have fewer than nine easy questions, weight the questions accordingly.)

That way, only students who are genuinely unprepared typically get less than a B (there are always more of these than I naively expect), but only very well prepared students get a high A, and you have some wiggle room to make adjustments as needed.