曲线y=f(x)和x=a，x=b及y=0所围成的图形的面积是定积分．(    )

单项选择题公开发行股票应由( )承销,承销包括包销和代销两种。

判断题若f(x)，g(x)都在[a，b]上可积，且f(x)＜g(x)，则当a＜b时，必有∫a→xf(t)dt可导．( )

判断题The Letter of credit shall be examined only by the exporter.

判断题Offer and counter-offer are two indispensable steps to reach an agreement.

判断题Acceptance should be declared only by written form.