Each of 12 refrigerators of a certain type has been returned to a distributor because of the presence of a high-pitched oscillating noise(高音调振荡噪声/高音調振盪雜訊/高音振動ノイズ) when the refrigerator is running. Suppose that 5 of these 12 have defective compressors and the other 7 have less serious problems. If they are examined in random order, let $X$=the number among the first 6 examined that have a defective compressor. Compute the following:

• (a) $P(X=1)$
• (b) $P(X\geqslant 4)$
• (c) $P(1\leqslant X\leqslant 3)$