When R$0$ is $0$x$20000000,$ examine each of the following instructions if its addressing mode is correct or not. State the reasons.

a$)$ LDRB R$1,[$R$0,$ #$1]$

b$)$ LDRH R$1,[$R$0,$ #$3]$

c$)$ LDRD R$1,$ R$2,[$R$0,$ #$4]$