Question 1 answer: A
Question 2 answer: H
Question 3 answer: J
Question 4 answer: T
Answer:
the first thing you do is do your experiment then title it. then state the purpose of the experiment. included a summary of the experiment. make a list of the materials you used. present all the steps in order to make the experiment possible. note any changes to the original procedure. this is basically the steps you have to do in order to make your scientific experiment.
Answer:

Explanation:
Given,
The density of Helium is 
We need to find the density in Dg/μm
We know that,
1 g = 10 dg
1 cm³ = 10¹² μm³
So,

So, the density of Helium is equal to
.
The chain reaction is easy to stop. Just add a neuron absorbing material. The Control Rods in rectors can do that You just SCRAM (put the rods all the way in) or add something like Boron and the chain reaction stops.
<span>The problem is the radioactive waste. Those isotopes break down and release heat spontaneously, no neutrons required. The only known way to stop or slow radioactive decay down is to slow time down by moving at relativistic speed or near orbit to a black hole.</span>