One of the keys to writing a successful questionnaire is to

a. ask yes or no questions.

b. ask exactly what you need to know.

c. use clever questions that hide your true motives.

d. provide multiple-choice answers for each question

i believe the answer is:   psychological  experiment

in this experiment, wilhelm wundt became the first person that implement scientific method in analyzing the pyschological phenomenon.

due to this result, wilhelm wundt also able to make psychology to be acknowledged as a separate scientific field from biology.

answer; the intervening variable;

answer; ancient times;

answer; observation and imitation;