[单项选择]下列程序执行后，变量a的值为( )。
Dim a,b,c,d as single
a=100
b=20
c=1000
if b＞a Then
　　d=a:a=b:b=d
End if
if c＞a Then
　　d=a:a=c:c=d
End if
if c＞b Then
　　d=b:b=c:c=d
End if
A. 0
B. 1000
C. 20
D. 100

In China phone talk is expensive. If you want to make it cheaper, make phone calls using the Internet, which can cost as little as 30% or even 5% of a traditional call.
But changing your DDD (domestic direct dial) and your IDD (international direct dial) calls for the net version, called voice over the internet protocol (互联网协议) or VoIP, requires you to choose between different types of technology, with different advantages.
If a telephone connection was a road, a regular phone call allows you and the person you are calling to share the road without anyone else. On this empty road, the connection is fast, efficient, and uninterrupted. But with VoIP technology, your voice is broken into millions of separated packets of information that share the road with all the billions of other bits of information on the Internet. When the packets reach the person you are calling, they are gathered to resemble your voice. Depending on which type of VoIP you are using, the
A. to introduce a new technology called VoIP
B. to inform that IP phone calls are sometimes quite awful
C. to tell us that IP calls are dependent on personal computers
D. to compare the IP calls with the traditional phone calls