苛勒的学习理论被称为（）。

A．试误说

B．经典条件作用

C．操作条件作用

D．顿悟说

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