Answer:
I feel it's to easily differentiate the positive from the negative...... like in gram staining of bacteria, the colour tell us which bacteria is gram positive or gram negative...... I just feel that this is science and it is advisable to know what the opposite result of an experiment should look like.
The answer is microscopic life appeared on Earth before flowering plants. This is because according to the label on the second box, microscopic life appeared 4.0 to 3.5 billion years ago, and this is way before the date 250 to 120 millions years ago indicated on the label on the sixth box the flowering plants appeared on Earth.
I think it is a petrified fossil but you didn't give any choices
