Why is it advantageous for scientist to test only one variable at a time during an experiment?

Sea stars, sea urchins, sand dollars, and brittle stars are all memeber of a group, what is the common name of the group they are all in

If two parents who's blood types are both b produce a child that has blood type o, what can you say about the parent's genotybes? oa) one is ii ob) one is ibi oc) both are bi od) nothing, this is impossible

When analyzing tree rings, what do scientists assume a thin ring indicates?