Some scholars have raised doubts about the genuineness and inspiration of Mark 16:9-20. With regards to that, I found an address entitled 'Completing Matters' by Mr J. Taylor (Vol. 78, p 67) helpful. Here's an extract: "I wish to speak at this time about the need for the completion of matters in the assembly. This thought has come to my mind in connection with the scriptures read, while in this meeting. There are persons who question the authenticity of these verses, claiming that they have been added to the book by someone other than the writer of the gospel. But they really are necessary to bind up, or complete, the gospel, and there hardly can be any doubt but that they are a part of Mark's writing. Mark has his own particular way of expressing himself. One thing that characterises him is that he is very brief — more so than the other gospel writers. But he is most careful to carry matters through to completion. And this is very important, and especially so in assembly matters. The Scriptures tell us that the creation was finished: "And the heavens and the earth and all their host were finished", Genesis 2:1. And we are also told how the matter was completed. The heavens were garnished, we are told. That suggests adornment — not only that they were finished, but that they were made beauteous. And Noah finished the ark — it was completed. The word is, "And Noah did it", Genesis 6:22. And so throughout the Scriptures we find incidents in which matters are brought to completion. God would have it so." I would highly recommend reading the full address, which can be found in full online.