b)

Кратко: 1 — since; 2 — for; 3 — for; 4 — since; 5 — since; 6 — since; 7 — for; 8 — for.

Правило. Since ставим, когда называется точка отсчёта — момент, с которого началось действие: since 2014, since last summer, since she was six. For ставим, когда важно сколько времени длится действие: for three hours, for four hours, for ages.

1. If the starting point is given (two o'clock, last Friday) we use since.
2. If the period of time is given (twenty minutes, one year) we use for.
3. My dad has been talking to them for three hours. — длительность.
4. Fiona and Fred have been learning French since 2014. — точка отсчёта.
5. I haven't been on holiday since last summer. — точка отсчёта.
6. Sasha has had freckles since she was six. — точка отсчёта.
7. I haven't eaten anything for four hours. — длительность.
8. I've known Lucy for ages. — длительность; for ages — устойчивое выражение.

Проверка: если можно спросить «since when?» — ставим since; если «how long?» — for.