Short Answer : C,D#,F#,A Works Fine!

Re. Buddy's posting.......
Quote "However, when I seek to play the key of C by pressing down the C1 and C2 notes together, the key of C7 shows on the LCD - producing a different sound of C7 whereas my intention of pressing the two C notes is to produce the sound of key C."

First, I assume that when you say 'the key of C', you mean the Chord of C.........

That is exactly how the 'Basic' system is supposed to work See page 54 of the manual. In Basic mode, a C7 chord will be produced by pressing note C2 and any other WHITE key below C2.....
If you wish to have both C2 and C1 sounding, then choose the 'Fingered' option. However, you will only hear these two notes if you have a LEFT sound selected and turned on.