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&e