[Javascript] Hide or enable a form element

[Imants Cekusins]

Is there a simple way to show/hide or enable/disable a <select> form element
by using another <select> element?

let's say there's a drop down list box F with 1) "weekly" and 2) "monthly"
options.

depending on which option is picked in box F, web page visitors would be
able to choose either from "week 1", "week 2", etc list (box W) or "Jan",
"Feb" etc list (box M).  The irrelevant box would then either become hidden
or disabled ("blurred").

A solution using drop down lists (<select>) rather than radio or check boxes
would be appreciated.