[求助]00-08-25 97-01-08(NA) 97-05-11 help!1 plz! thank u

1. Visibly only through large telescopes,Pluto has a yellowish color,which indicates that there is very little atomosphere.

( i don't know how to do this one, and can u tell me why ? why adv. can be subject?)

2. Volcanoes are divided into three main groups, based on their shape and type of material ,they-----

A are made

B. made of

C. are made of

D made for

In this sentence, BASED is a verb ?  the compelet sentence is this or not?

(Which based on their shape and the type of material that they-----)

7. With age, the mineral content of human bones decreases,----thm more fragile.

C thereby making

( why here put thereby, what's it mean?)

1. visibly>>visible，前面省略了being，

2. C， Which are based on their shape and the type of material that they-----

7. 随着年龄的增长，人类骨骼的矿物质含量减少，因而（thereby）导致骨骼更加脆弱。