Thickness measurement apparatuses 1 and 2 are set a preset distance Ld from each other. In various embodiments, the present disclosure relates to methods for inhibiting polymerization of vinyl aromatic compounds during extractive distillation. A front end of the small memory card receiving member is located at a position more rearward than a front end of the large memory card receiving member. A wiper is mounted on the front of the ram, and the wiper has a plurality of teeth located in respective grooves in the plate. A control unit controls a switching between a first transmission frequency band FC1 and a second transmission frequency band FC2, wherein the control unit changes correspondingly and simultaneously to the switching in the first transmission frequency band FC1 and the second transmission frequency band FC2 between a first and a second channel bandwidth.