一艘宇宙飞船绕一个不知名的行星表面飞行，要测定该行星的平均密度，仅仅需要（　　）

A．测定飞船的运行周期

B．测定飞船的环绕半径

C．测定行星的体积

D．测定飞船的运行速度

In the foreign trade business, the payment is always made by L/
C．In this sentence, L/C is the abbreviation for ______.

A．bill of Lading
B．commercial Invoice
C．buying Contract
D．letter of Credit