A、6.5. B、5.85. C、5 D、19 A从“The average decrease is 6．5 percent a year since 1990．”可推知。

M: Sorry to be calling you so late but I’m having trouble with the math assignment. And I am sick of doing it alone.
W: Oh,.. .well, do you want to come over? But I do have to say that I’m still not halfway through it.
Q: How does the woman respond to the man’s problem?

W: Anything new in today’s paper?
M: Yes. The infant mortality rate in China has declined notably since the beginning of the 1990s. The average decrease is 6.5 percent a year since 1990. The mortality rate of children under five years old has been tailing off by 5.85 percent annually.
Q: What is the percentage of the annual decrease in the infant mortality rate since 1990?