barplot in R programming language : I have this package called gapminder.

I am trying to do this

a.	A bar chart showing the life expectancy for the United States over the years. (Data source: gapminder)

Make a barchart showing the life expectancy of the United States over the years. But I am not entirely sure how to do this.

Can anybody show me how to do this I did names(gapminder) and I get
"country"   "continent" "year"      "lifeExp"   "pop"       "gdpPercap"