Which of the following statements about market efficiency is true?
a) A market can be semi-strong-form and strong-form efficient but not weak- form efficient.
b) A market can be semi-strong-form efficient but not weak-form efficient.
c) A market can be semi-strong-form, weak-form or strong-form efficient but not all simultaneously.
d) A market must be weak-form efficient if it is semi-strong-form efficient.