To round is to delete or (66) one or more of the least significant digits in a positional (67) and to, or not to, adjust the part retained in accordance with some specified rule. The purpose of rounding is usually to limit the (68) of the numeral or to reduce the number of characters in the numeral, or to do both these things.

The most common arithmetic forms of rounding are rounding down, rounding up and rounding off. Rounding down is defined as making no adjustment to the part of the numeral that is retained. Rounding up means adjusting the part of the numeral that is retained by adding 1 to the least significant of its digits and executing any necessary carries, if and only if One or more non-zero digits have been deleted. Rounding off (69) adjusting the part of the numeral retained by adding 1 to the least significant of its digits and executing any necessary carries, if and only if the most significant of the digits deleted was equal to or greater than half the (70) of its digit place.

（66）处填（）。

A．simplify

B．reduce

C．abbreviate

D．omit

固体培养基使用下列哪种物质作为凝固剂最好：（）。

A.明胶

B.琼脂

C.果胶

D.纤维素