I know 和 I know it 都可以,虽然不用it的情况更多一些。此题不够严谨。
《韦氏高阶》:
I know (it)
1 — used to express agreement
“Hurry up, we're going to be late.” “I know, but I can't find my shoe.”
“This place is such a mess.” “I know it.” [=I agree]
“I can't believe he lied.” “I know, I know.”
“The whole situation is just so stupid.” “I know.”
 
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