Fire and Mello conducted an experiment where they added no gene X RNA (b), anti-sense gene X RNA (c) or double stranded gene X RNA (d) to C. elegans embryos. They then did a hybridization experiment examining gene X mRNA levels in the embryo. Square (a) is a no probe control. How does this act as a control

The correct answer is - It shows that an RNA probe is necessary to get a signal.

Explanation:

A probe is a specific single-stranded sequence of DNA or RNA that recognizes its complementary sequence in a particular sample genome for synthesis. The RNA probe is placed into close contact with the given sample under a specific environment that allows the RNA probe sequence to hybridize with its complementary sequence.

In this case, square a is the condition where no RNA probe is persent whereas all three other conditions have some sort of RNA probe, however, in C there is no Gene X probe but their probe present. To get a signal in hybridization RNA probe is required and that can be used as control as it can be used to compare if a signal is received or not.

the fossil record provides evidence of a common ancestor to many species.