na.rm in sd()

Hello, I think this qualify as a bug

 > x<-c(1,2,3,4,NA,6,7)
 > mean(x)

[1] NA

 > mean(x,na.rm=T)

[1] 3.833333

 > sd(x)

Error in var(as.vector(x)) : missing observations in cov/cor

 > sd(x,na.rm=T)

Error in sd(x, na.rm = T) : unused argument(s) (na.rm ...)

 > var(x)

Error in var(x) : missing observations in cov/cor

 > var(x,na.rm=T)

[1] 5.366667

 >

why sd() does not recognize the na.rm=T parameter, while var does?

args(sd)