Which C++23 statement about shared_ptr and threads is correct?

Which C++23 statement about shared_ptr and threads is correct?

1. Different shared_ptr objects may share a control block across threads, but the managed object still needs its own synchronization.
2. Reference counting makes every field of the managed object atomic.
3. Two threads may call reset concurrently on the same ordinary shared_ptr variable without synchronization.
4. weak_ptr::expired keeps the target alive until the next statement.