在共點(diǎn)力的合成實(shí)驗(yàn)中,如圖所示,使彈簧秤b按圖示位置開始順時(shí)針方向緩慢轉(zhuǎn)動(dòng)90°角,在這個(gè)過程中保持O點(diǎn)位置不動(dòng),a彈簧秤的拉伸方向不變,則整個(gè)過程中關(guān)于a、b彈簧秤的讀數(shù)變化是(   )                                       

                                                                                                         

    A. a增大,b減小         B. a減小,b減小

    C. a減小,b先減小后增大 D.      a先小后增大

                                                                                                                                    

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