Read the sentence how the fire started is still unknown which kind of clause are the words in bold a. adverb clause b. noun clause c. adjective clause

martians or other types of aliens represent a unique fear as opposed to zombies. vampires and other types of monsters.   in most sci-fi stories, aliens come from other worlds and have different or more advanced technology.   abductions and experimentation on human bodies that involve these technologies, or the use of these technologies against human beings and the use of unknown "powers", are a type of fear that's different from that of other types of monsters.  

the correct answer is the first one.