Which of these lines provides important information for the resolution of the play?
a. O Banquo, Banquo!/Our royal master's murder'd!
b. To crown my thoughts with acts, be it thought and done:/The castle of Macduff I will surprise
c. Hail, king, for so thou art: behold, where stands/The usurper's cursed head:
d. And, for an earnest of a greater honor,/He bade me, from him, call thee Thane of Cawdor