Fifty Milliseconds Too Late

Three clients operate on one register x, initially 0. Times are milliseconds on a single wall clock the observer trusts; each row shows when the client sent the request and when it got its response.

client	operation	sent	responded	result
A	`write(x, 1)`	0	40	ok
B	`read(x)`	50	60	`0`
C	`read(x)`	70	80	`1`

How should this history be classified?

Linearizable if B and C were talking to different replicas, and not linearizable if they shared one.

Sequentially consistent but not linearizable.

Linearizable, since a valid order exists in which B's read precedes A's write.

Neither, because B returned a value that a completed write had already replaced.