Answer: Some think it is altogether too difficult.
Explanation: The word "altogether" is an adverb. Moreover, its meaning is "completely" or "considering everything". "Altogether" is the word that completes the sentence provided because it is coherent or shows a clear relationship with the idea expressed in this sentence. In contrast, "all together" is formed by two words and it has a completely different meaning. "All together" is generally used to mean "with each other". In that way, "all together" does not add coherence to the sentence provided; therefore, the correct option is "altogether".