Though this adventure follows a fairly set story line, part of your job is to mask this from the players. When only one choice out of several will advance the PCs along in the adventure, use your wiles to subtly make that choice the most logical, most appealing one for your players. Only you can do this, as only you know how to best tantalize your particular group of gamers. This is a test of your skills as a DM—to keep your players interested and to make them believe that they are free to make any choice they wish.