18℃时,H₂A(酸) :K1=4.3×10^-7,K2 = 2.1×10 ^-12 ,H₂B（酸）：K1=1.0×10^-7,K2 =6.3×10^-13，在浓度相同的两种溶液中，用“>”“<”或“=”填空：

(1)H⁺的浓度：H₂A____H₂B;

(2)酸根离子的浓度：c(A^2-)____c(B^2-)；

(3)酸分子的浓度：c(H₂A)_c(H₂B)；

(4)溶液导电能力：H₂A____H₂B。

Electronic mail, or mail, as it is known to its many fans, has been around for over two decades. Typically, email systems support five basic functions, as described below. (41) refers to the process of creating messages and answers. (42) refers to moving messages from the originator to the recipient. (43) has to do with telling the originator what happened to the message. Was it rejected Was it lost (44) incoming message is needed. So people can read their email. (45) is the final step and concerns what the recipient does with the message after receiving it.

（45）是（）

A.Composition

B.Terminating

C.Reporting

D.Displaying