Algorithm time complexity problem

Four computer science professors are discussing the time complexity of an algorithm. Professor Turing says the time complexity is O(n^5), Professor Knuth says it's O(n^4), professor Hopper says it's O(n^3), and professor Dijkstra says it's O(n^2). All of them claim their estimates of time complexity are tight. Alexandra, an enterprising student, codes, runs, and times the algorithm with various input sizes. Here are the results.

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http://s1002.photobucket.com/user/imabeardeddragon/media/FullSizeRender_1.jpg.html?o=0

Based on these results, which of these professor's estimate is correct? Explain your answer using the definition of Big O from part a.