"The space shuttles is the subject."
Simple formula: find the verb of the sentence then ask the question "who or what performs the verb" and the answer to that question is the SUBJECT of that sentence!
In your sentence; ask the question;
"Who or what have made travel to the International space station convenient and possible?
Answer: The space shuttles (that's your subject)
C. Characterization.
"Characterization is a literary device that is used step-by-step in literature to highlight and explain the details about a character in a story. It is in the initial stage in which the writer introduces the character with noticeable emergence. After introducing the character, the writer often talks about his behavior; then, as the story progresses, the thought-processes of the character."
