retrieves the user-entered radius from the radius input field into a variable called sphere_rad,
writes the value of sphere_volume into the area input field.
Test your page by entering r = 1. As output you should get 4.19 ...
3. [Extra Credit] Write a currency converter that allows you to specify the type of conversion (e.g. Euro->Dollars, Dollars->Euro, Yen->Euro, etc.) using a drop-down list of options. Clicking a button should then perform the currency conversion (using form input fields for input and output). Your converter should support conversions back and forth between three currencies, you can pick which (this means your drop-down list will have six choices). Hint: it's not as difficult as it may seem; in the option tag you can actually specify a value which is different from the choice that appears in the drop-down list. That'll come in very handy.