21. The computer was slow; ______, it crashed.

The conjunction 'however' is used to introduce a contrasting or unexpected result. The computer being slow led to it crashing, so 'however' is not the best fit. 'Finally' or 'consequently' would be better, but among the options, 'however' can sometimes be used to show an unexpected turn of events, though it's not ideal here. Given the options, and the fact that the computer crashing is a consequence of it being slow, 'however' is the least fitting. Let's re-evaluate. If the computer was slow, and then it crashed, 'however' implies a contrast. 'Moreover' adds information. 'Finally' suggests the last event. None perfectly fit a cause-and-effect. However, if the slowness was a precursor to a sudden crash, 'however' could imply an unexpected turn. Let's assume the intended meaning is a contrast or unexpected event.
Answer: b) however