I mark all the questions but do not give them grades. This works much better than one longer question at the beginning - it really helps keep the students focused on getting through the investigation during the class. And finally I bring all the groups together for the last ten minutes to answer their questions from the investigation. One advantage of having the students work in groups is that when a student asks a question it implies that nobody in his group knows the answer - he's not the only one. Now there are always tradeoffs, of course, to whatever methods are used to teach classes.
Most students come to class with a reasonably good start on the day's material.
They may not have all the answers but they typically have a pretty good idea of where they need help. Students are directly participating in the development of the class results. Students have ample opportunity to ask questions of their group and the instructor. The instructor gets detailed information on how well the students are understanding the material by how well they're doing the investigations. And so the instructor gets detailed information on what needs improving.