R ships with several built$-$in datasets, including the famous iris $($flowers$,$ not eyes$)$ data

collected by Anderson and analyzed by Fisher in the $1930$s$.$ Type iris to see the dataset.

a$)$ Create a new data frame that consists of the numeric columns of the iris dataset and

calculate the means of its columns.

b$)$ By using aggregate function, calculate the mean and variance for variables sepal

length, sepal width, petal length and petal width according to species.