one question, dr. wei.
could i ask you a question, prof. li?
may i venture to ask prof. zhang a question?
before asking your question, you can express your positive attitude or make a comment on the speaker's presentation. for example,
dr. johnson, i was fascinated by your description of your study, but what will happen if...
mr. li, you did splendid work! just one question.
you mentioned very briefly that you used two experiments that were the same.
would you please elaborate on that point?
would you be so kind as to give me more information about the method of your experiment?