Four machines are set up to produce alloy spacers for use in the assembly of microlight aircraft. The spacers are supposed to be identical, but the four machines give rise to the following various lengths in mm. Using 5% level of significance, the mean lengths of the spacers using four machines differ. 

여기서 마지막 문장 무슨뜻으로 알아 먹어야하냐??

differ뒤에 온점이 맞음? 물음표아니야?