Answer:
Dr Hewitt is able to break the piece of wood in his demonstration given that he had knowledge about the concept of momentum, which is esentially the product of the mass and speed of an object.
Aplying this idea, he used the correct momentum of his hand over a short period of time and that is how he was able to break the piece of wood.
The correct sentence is A.
All of the other sentences are grammatically incorrect.
Hope I helped!
The most credible source in terms of researching the effects of smoking is letter b, research article in a scientific journal. It is because a credible source means that it is valid or could be relied on in which the research article is, because it has been researched and holds validity of which contains proof. Other choices does not correspond to the question for they don't hold any validity or proof in the topic that could be reliable.
