Answer: One sign of physical change is melting, freezing.
The answer is B, because cells make up tissue, tissue makes up organs, organs make up the organ system, and an organ system makes up an organism. Imagine it as a process from least to greatest complexity.
<span>180 degrees, 120 degrees.
The Carbon are all in a straight chain, with the Hydrogen at angles at the end.
From VSEPR theory, the Carbons bond in an AX2 system, and the H C H bond is an AX3 system, which provide the two different angles.</span>