对于受要约人延误的承诺，该承诺( )。
A．有效
B．要约人虽表示接受也无效
C．无效
D．要约人未表示不接受有效
参考答案：D
血铅水平分五级，I-Ⅳ级属于不同程度的铅中毒。
How can a SART’s effective range be maximized? （）
A.The SART should be placed in water immediately upon activation
B.The SART should be held as high as possible
C.Switch the SART into the "high" power position
D.If possible，the SART should be mounted horizontally so that its signal matches that of the searching radar signal
