在化学反应4H₂O+3Fe═Fe₃O₄+4H₂中，发生氧化反应的物质是（　　）

A．H₂O

B．Fe

C．Fe₃O₄

D．H₂

The nurse is performing a painless, noninvasive procedure to measure arterial oxygen saturation (SaO₂). What procedure is it（）

A. Incentive spirometry.

B. Arterial blood gas (ABG) measurement.

C. Peak flow measurement.

D. Pulse oximetry.