單項(xiàng)選擇題當(dāng)“窄”發(fā)光二極管亮?xí)r,單脈沖觀察孔輸出已經(jīng)調(diào)制的單脈沖,雙脈沖觀察孔輸出相位差為的雙脈沖()

A.450
B.300
C.600
D.不定