Exactly six lectures will be given one at a time at a one-day conference. Two of the lectures—S and T—will be given by resident speakers, the other four—W, X, Y, and Z—will be given by visiting speakers. At least two but no more than four of the lectures will be given before lunch; the remaining lectures will be given after lunch.
The following conditions must be observed:
S will be the fourth lecture.
Exactly one of the lectures by a resident will be given before lunch.
Y will be given at some time before T is given.
If W is given before lunch, Y will be given after lunch.
Which of the following can be the order of lectures
and lunch at the conference?
If exactly two lectures are given before lunch, they
must be ：
If exactly three lectures, including Y and Z, are given
before lunch, which of the following can be true?
If T is the sixth lecture, which of the following must
be true?
If S and Z are both given after lunch, which of the
following must be true?
Which of the following lectures CANNOT be given
immediately before lunch?