答案:A
---I don’t know how to write the article assigned by our professor on international trade.
--You should have ______ your attention on the lecture that ______ how international situation affects exports and imports across the globe.
A.put; concentrate on
B.paid; talked about
C.fixed; dealt on
D.focused; focused on
按折射率分布划分,光纤可分为()和渐变型。