** **In this exercise, you will work with rivers data. The data pertain to the length of major North American rivers, and it is built-in data in R. You can access the data by typing:

> rivers

Your task is to build a function that returns descriptive statistics for different inputs. The function is named descriptive_function and has three arguments (i.e., another name of inputs).

descriptive_function(list, char, num)

The requirements of the function are as follows:

· The function has three arguments. The first argument is a list of numbers in doubles. The second argument is a character, and the last argument is a number.

· If char is ‘m ’, the function prints the arithmetic mean of the list.

· If char is ‘v’, the function prints the variance of the list.

· If char is ‘s’, the function prints the standard deviation of the list.

· If char is ‘z’, the function prints the z-score of num with respect to the list.

· You can use built-in R functions including mean(), var() and sd().

**Sample test cases:**

> descriptive_function(rivers, ‘m’, 7)

[1] 591.1844

> descriptive_function(rivers, ‘m’)

[1] 591.1844

> descriptive_function(rivers, ‘s’)

[1] 493.8708

> descriptive_function(rivers, ‘v’, 8)

[1] 243908.4

> descriptive_function(rivers, ‘z’, 8)

[1] -1.180844