45. 如图,直三棱柱ABC−A1B1C1ABC-A_{1}B_{1}C_{1}ABC−A1B1C1中,AC⊥BCAC\bot BCAC⊥BC,AC=BC=CC1=2AC=BC=CC_{1}=2AC=BC=CC1=2,MMM,NNN分别为ACACAC,B1C1B_{1}C_{1}B1C1的中点.(Ⅰ)求证:MN//MN//MN//平面ABB1A1ABB_{1}A_{1}ABB1A1;(Ⅱ)线段CC1CC_{1}CC1上是否存在点QQQ,使A1B⊥A_{1}B\botA1B⊥平面MNQMNQMNQ?说明理由.