Multisig Who puts second signature into the script? I know how the script work together, but the Multisig is still confuse me. 

input script:
OP_0 [A's signature] [B's or C's signature] [serialized redeem script]

how the transcation being created,
For example, "A" create a transaction, he can put a signature into the script, but where is the B signature from? A's wallet definally no B private key!

Logically, the transaction must send to B, then B put a signature into the script, right?
After "A" signs the transaction hash the hash gets updated to show his signature. "A" will send this partially signed transaction hash to "B" who will then sign that hash which will then return a third hash reflecting two valid signatures. Assuming only 2 signatures are required to spend, this third hash is what gets broadcast to the network.