traffic<-read.csv('https://raw.githubusercontent.com/kunal0895/RDatasets/master/TRAFFIC.csv')
 D<- mean(traffic[traffic$TUNNEL=='Holland',3]) - mean(traffic[traffic$TUNNEL=='Lincoln',3])
 null_tunnel <- rep("Holland",2801)
 null_tunnel[sample(2801,1400)] <- "Lincoln"
 null <- data.frame(null_tunnel,traffic[,3])
 names(null) <- c("TUNNEL","VOLUME_PER_MINUTE")
 summary(null$TUNNEL)
 holland_null <- null[null$TUNNEL == "Holland",2]
 lincoln_null <- null[null$TUNNEL == "Lincoln",2]
 mean(holland_null)
 mean(lincoln_null)
 D_null <- mean(lincoln_null) - mean(holland_null)
 D_null
 D