polygon() Function

a. Run the following commands:

x <- c(1,2,3,4)
y <- c(0,1,1,0)
plot(x,y)
polygon(x,y,col="blue")

What do you see?
A blue pentagon
A blue rectangle
Two blue right-angled triangles separated by a white rectangle
A blue trapezoid with vectices at (1,0), (2,1), (3,1) and (4,0)

b. Run the following commands:

x1 <- -1
y1 <- 0
x2 <- 1.5
y2 <- 0
xcurve <- c(-1,0,1.5)
ycurve <- dnorm(xcurve)
x <- c(x1, xcurve, x2)
y <- c(y1, ycurve, y2)
curve(dnorm(x), xlim=c(-3,3))
polygon(x,y, col="gold")

What do you see?
The standard normal curve between x = -1 and x = 1.5 is missing and is replaced by a 5-sided gold polygon.
A 5-sided polygon shaded with gold color with 2 vectices on the x-axis and 3 vectices on the standard normal curve.
Only the standard normal curve is plotted.
An error message: "gold" is an invalid color.
No plot is shown.

c. Run the following commands:

xstart <- 1.2
ystart <- 0
xend <- 4
yend <- 0
xcurve <- seq(xstart, xend, length.out=100)
ycurve <- dnorm(xcurve)
x <- c(xstart, xcurve, xend)
y <- c(ystart, ycurve, yend)
curve(dnorm(x), xlim=c(-xend,xend))
polygon(x,y, col="skyblue")

What do you see?
The region above the standard normal curve with 1.2 < x < 4 is shaded with skyblue color
The region under the standard normal curve with 1.2 < x < 4 is shaded with skyblue color
The region with 1.2 < x < 4 (both above and below the curve) is shaded with skyblue color
The region under the standard normal curve with -4 < x < 1.2 is shaded with skyblue color
The region under the standard normal curve with -4 < x < 4 is shaded with skyblue color