Diatomaceous earth is used as a heat insulator in boilers and steam pipes because the cell wall of diatom is
1.  Composed of iron
2.  Composed of silicon dioxide
3.  a good conductor of heat
4.  a bad conductor of electricity

Auxospores and hormocysts are formed respectively, by:

1. Some cyanobacteria and many diatoms

2. Several diatoms and a few cyanobacteria

3. Several cyanobacteria and several diatoms

4. Some diatoms and several cyanobacteria