music theory

Yes, 1=D is D major, which means D is 1.

C, D, D, E, E, F, G, A, B, B are only half a degree apart. You can work out the problem yourself.

Look at the keyboard. Every two black keys lean together, then every three black keys lean together, and then cycle again ... The white key between two black keys lean together is D, black keys on the left of D is C, starting from C, the white key is CDEFGAB in turn, and black keys is the rise or fall of the white key on the left.