Neurotransmitters are chemical molecules released by the neurons to bridge the gap between and to be able to communicate. This is a process in which one side of the communication is the source of the neurotransmitters and the other is the receptor which is often used as a lock-key example. Since the neurotransmitter is the component which bridges the gap and in a sense unlocks the door, it is the key. The receptor is the part affected by the neurotransmitter which means that is the lock part of this example.
DNA, or deoxyribonucleic acid, is a long chain molecule that plays a central role in life on earth. The information encoded in strands of DNA controls the genetic makeup of organisms. The DNA molecule has a backbone of sugars and phosphate groups off of which hang simpler units called nucleotide bases.
In a scientific experiment, the dependent variable is the variable that is being tested and then measured. The answer referenced in D, the variable that is manipulated, is referring to the independent variable.
