It suddenly ______ to the detective that the professor was probably murdered by his own wife .

A．came about

B．took place

C．happened

D．occurred

U₁N、U₂N分别为三相变压器原、副边额定电压，I₁N、I₂N为原、副边额定电流，下列各式中①S=3U₁NI₁N，②，③，④S=U₁NI₁N，哪一些可用于计算三相变压器容量S的公式（）。

A．①

B．④

C．①④

D．②③