请至少说明GIF格式的两种特点：（）；（）。
参考答案：无损压缩，256色，动画，透明色。4选2即可。
用括号中所给词的适当形式填空。
1. It is our duty (keep) our classroom clean.
2. Don't worry. There (be) plenty of time.
3. The students' (pronounce) isn't good, but it's improving.
4. John is worried about the coming (final) exam.
5. There must be something (visit) my home.
在重铬酸钾法测定COD时，用（）作指示剂。
A.试亚铁灵
B.甲基橙
C.EDTA
D.淀粉
