Yuri created a formula for a button. The code is supposed to set the value of an existing field called title. He used the following formula: title:= @propercase (title); after the foumula runs ,the title field is null. Why? （）

• A、the value of one field cannot be set from the value in another field 
• B、the text string in the formula is set to null. Thus notes cannot validate the formula 
• C、when setting the value of a field , the formula must be in the field‟s input validation event 
• D、the formula does not specify that title is an existing field. Thus it is interpreted as a temporary variable