Most people are familiar with the general structure of a hen's egg. Its outer covering is known as the shell. The shell is hard and easily breakable. It is made of a chalky substance known as carbonate of lime or calcium carbonate. It is a white-colored, natural substance that can also be found in chalk, limestone, and marble.
If you put some pieces of an eggshell into a cup and pour over them a little vinegar, you will see them melt gradually. Then, small bubbles of gas will rise into the air. But if you take a long and narrow strip of the shell and heat one end of it, after a short time, that end will become softer. It will also glow brightly. This happens because the shell is converted into lime. Lime is the substance builders use to make cement. This bright glow is really a light, which is always produced when a piece of lime is heated.
Just inside this eggshell is a thin membrane, or skin, that can be easily peeled off. Next to this, comes the "white" of the egg. It is a colorless liquid but turns white and more or less solid or firm while cooking. Last of all, in the center of this egg sits an oval, yellow mass that is called the "yolk."
Now, imagine this egg is reduced in size and the outer shell is entirely removed. The skin or membrane is converted into a firmer substance of a rough nature, and finally, the yolk is absent. This egg would then represent the egg of a butterfly or a moth.
The eggs of these insects are simply little, liquid masses, usually of a colorless substance, which have a rough covering surrounding them.



Which sentence best shows an idea that is important to include in a summary of this passage?
A.
Then, small bubbles of gas will rise into the air.
B.
This bright glow is really a light, which is always produced when a piece of lime is heated.
C.
Next to this, comes the "white" of the egg.
D.
It is a colorless liquid but turns white and more or less solid or firm while cooking.