1. Why are there missing keys with 2 threads, but not with 1 thread? With 1 thread, the put and insert are always interleaved. No changes to hash table happen at the same time. But with 2 threads, there will be race to access the hash table, and this leads to the missing keys. 2. Identify a sequence of events with 2 threads that can lead to a key being missing. Suppose thread No. 1 is running the insert and so does thread No. 2. They shall both insert a entry. But because they enter the insert function with the same hash table, they could only insert one key at most, which causes a key being missing.