Thanks, Ge!
I did some home works. See this:
"For the inner planet, it never gets very far away from the Sun. The further in the planet is, the closer to Sun (in the sky) it is. There is a maximum angle between the Sun and the planet in the sky and that happens when the planet subtends a right angle between Earth and the Sun. This is called the greatest elongation and in the case of Venus, it is something like 46 degrees. Hence, the maximum Venus can get from the Sun (either in the morning sky or the evening sky) is 46 degrees and hence
Venus always sets in less than about 3 hours after sunset or rises no earlier than 3 hours before sunrise.
This is also the reason why Mercury is so hard to see; it is so close to the Sun that its maximum elongation is only about 19 degrees and so sets before an hour after sunset or rises no earlier than an hour before sunrise."
http://curious.astro.cornell.edu/question.php?number=177