Hi, I'm just starting to read Polchinski's book on string theory. I have the follow questions: 1. In the intructionary part, he said different kinds of string theory arise from different ground states(vacuum), but I don't understand why the difference on dimension of spacetime or open/closed or oriented/unoriented or choice of gauge group come from different vacuum. And does the "different vacuum" in string Landscape stand for the same meaning? 2. I don't understand chiral gauge couplings. Does "chiral" of gauge boson defined by parity asymmetric? But in QM it seems to stand for relationship between spin and moving direction. 3. I don't understand the Ployakov action. Indeed I can do the varition of the metric to see it is the same as Nambu-Goto action classicly, but I don't know how the Ployakov action is found. People just guess it is the form and prove it is so, or there is some formal way to take variable out of square root and get a quadratic action? And by the way, do N-G action and Ployakov action equal at the quantum level? Thank you very much and happy new year!