Hi there, (i) Could anyone tell me the difference between p-branes and D-branes please? (ii) I have recently read that p-branes are some higher dimensional cousins of the extremal Reissner Nordström black hole, what does that actually mean in type IIB string theory? (iii) If we have a U(2) gauge boson, does that mean it's a 2x2 matrix that is unitary? What other properties does this gauge boson have? Thank you to all!