下列各式中，正确的是[ ] A.x2y-2x2y=-x2y B.2a+3b=5a

网络协议主要由3个基本要素组成，即

A．层次、语义和时序

B．语法、原语和时序

C．语法、语义和时序

D．语法、语义和功能

使用灭火器扑救火灾时要对准火焰（）进行喷射。

A、上部 

B、中部 

C、根部

在研磨过程中起调和磨料、冷却和润滑作用的是（）

A、研磨液 

B、研磨剂 

C、磨料 

D、研具

癔症( )

A．深大呼吸(Kussmaul呼吸)
B．呼吸频率增快
C．潮式呼吸
D．呼吸慢而深
E．呼吸频率浅表而频数

On November 15, 2008, Grinell Construction Company decided to issue bonds to help finance the acquisition of new construction equipment. They issued bonds totaling $10000000 with a 6% coupon rate due June 15, 2028. Grinell has agreed to pay the entire amount borrowed in one lump sum payment at the maturity date. Grinell is not required to make any principal payments prior to maturity. What type of bond structure has Grinell issued（）

A. Serial bonds.

B. Bullet maturity.

C. Redeemable bonds.