In the flipped classroom, students learn on line by watching video madeby the teacher, then solve problems and apply the knowledge in class with thehelp of teacher.

Label each of the following as either a physical process or a chemical process:(a)rusting of a metal can is a process;(b)boiling a cup of wateris a process;(c)pulverizing an aspirinis a process;(d)digesting a candy baris a process;(e)exploding of nitroglycerinis a process.

In many places, tap wateris both clean and ____1__. In fact, there are more unhealthy chemicals in bottled water. These ____2__ come from the plastic. Here’s another fact: Most bottled water is the same as tap water. Both tap and ____3__ water come from rivers and lakes. But bottled water is more expensive. It costs almost a ____4__ times more than tap water. Putting water in bottles also wastes water. Water is used to clean bottles first, and this is ___5____. Of course, in some places, people need to drink bottled water. The tap water isn’t clean in those places. But this must ____6__. Eighty percent of plastic bottles go in the trash. And this is very bad for our ___7____.

20A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、120、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、130、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、A、B、C、D、140、A、B、C、D、A

以下集合A={a,b,c,d}上关系，其中（ ）是等价关系。 A: {(a,a),(a,d),(b,b),(b,c),(c,b),(c,c),(d,a) } B: {(a,a),(a,b),(b,a),(b,b),(c,c),(c,d),(d,c),(d,d)} C: {(a,a),(b,b),(b,c),(c,b),(c,c),(d,a),(d,d)} D: {(a,a),(a,b),(b,a),(b,b),(b,c),(c,b),(c,c),(d,d)}