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4. What Event(s) for a Textbox can determine when the text inside the control has been modified 5. What Event, Property, or Method can be used to make a Textbox disappear? a 6. How do you declare a variable of type Double and assign it a value? (Hint: use the C# code example provided in the class information) 7. What Event, Property, or Method from the System.Double structure can be used to convert a string of characters like "25.79123" to a System.Double? 8. What is the NaN field for the System.Double structure? 9. What Method of the String class could you use to join 3 separate Strings into one String? Does the method return a value? Give a code example on how to use the Method. You only need to show how to call the Method. 10. What Property of the CheckBox class can be used to determine whether or not the user placed a checkmark in the CheckBox? 11. What Property of the ComboBox class will return a string representing the user's choice from the list of things in the ComboBox? What Event from this class fires when the user of the control chooses an item from the ComboBox. NNNNNN