题目比Φ3mm平底孔回波小7db的同声程平底孔直径是:( )A. Φ1 nunB. Φ2mmC. Φ4mmD. Φ0.5mm比Φ3mm平底孔回波小7db的同声程平底孔直径是:( )A. Φ1 nunB. Φ2mmC. Φ4mmD. Φ0.5mm题目解答答案B. Φ2mm