To find proof as to whether he is "in love" or not, Yukimura pats Himuro on the head from point-blank range. Seeing this, Ibarada, an upperclassman in the Ikeda Lab, talks about the need for a control experiment. In other words, in order to prove that the racing of his heart is because of Himuro specifically and not just because he's facing a woman, Yukimura has to pat Kanade and Ibarada on the head too. Himuro, on the other hand, sets out to prove that food made with love tastes better and cooks, but...
Otherwise, you can't prove that 
the other party must be B.
وإلّا فلا يمكنك أن تثبت أنّ الطرف
.الآخر لا بدّ أن يكون ب
Ansonsten ist nicht beweisbar,
dass das Gegenüber zwingend B sein muss.
Sans quoi, vous ne pouvez établir
le rôle de B dans les résultats.
Altrimenti, non si può provare 
l'assoluta necessità che l'altro soggetto sia B.
Caso contrário, não temos como determinar
se só se aplica se o outro elemento é B.
Иначе никак не доказать, 
что А реагирует так только на Б.