Directions: Which sentence is different in context from the others?

(I) Flying saucers is the name given to saucer-like shapes which have, on occasion, been seen traveling through the atmosphere. (II) For some time speculation was rife, especially in America, as to what they were. (III) So it is time they were taken seriously and studied scientifically. (IV) Now, however, it is believed that they are nothing more than atmospheric phenomena. (V) That is to say they are as unreal as mirages or mock suns caused by unusual atmospheric conditions