A series of debates tetween the lectures was scheduled for the next weekA series of debates tetween the lectures （ ）scheduled for the next week.a was b were c have been d are为什么选a?next week 是将来时啊,was 是过去时啊.还有lectures 不是可数名词吗?但为什么是单数呢，lectures是复数啊

A series of debates tetween the lectures was scheduled for the next week
A series of debates tetween the lectures （ ）scheduled for the next week.
a was b were c have been d are
为什么选a?next week 是将来时啊,was 是过去时啊.还有lectures 不是可数名词吗?
但为什么是单数呢，lectures是复数啊