What it means is that he wouldn't have been able to get where he got if it hadn't been for the scientific achievements of his predecessors. If it hadn't been for these "giants" that enabled him to see so far, he wouldn't have been able to make all the discoveries that he did. For those coming after him, he was a giant that enabled them, and so laws and theories and scientific knowledge always increases based on the knowledge before it.
Plan your time out wisely,
utilize free time but often take breaks so you don't over work yourself. Make sure you understand your goals and how you much achieve them. Set your goals each day and how much you want to study and revise.
I would say d as they are all important
