Explain why not all microbes are harmful.

All the microbial activities and products are not harmful for humans. Hence all microbes are not harmful.

Explanation:

There are 2 types of microbes present around us- good microbes and bad microbes. The goodness and badness of microbes depend upon their activities. There are several viruses and bacteria which cause different diseases in the human body. At the same time, there are other kinds of microbes which are good for us.

The E.coli bacteria in our gut maintains the normal flora of our intestine and maintains a symbiotic relationship. The beneficial microbes maintain symbiotic with the host and their microbial products are useful.  

For example,  yeast is used in making cakes, breads, biscuits. The yeast powder is used for these purposes. Some microbial fermentation products are edible to humans. The lactobacillus bacteria help in the curdling of milk. Many antibiotics used for us comes from microbial activities. These are the reasons all the microbes are not harmful.

gold i'm pretty sure i might be wrong

chert is a sedimentary rock composed of microcrystalline or cryptocrystalline silica. with this little bit of information, you can come to the conclusion that your answer is c.  

explanation: