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【单选题】
下列命题中，真命题的个数为（ ） ①x 2 +bx+c=0有一根大于1，另一根小于1的充要条件是1+b+c＜0 ②当a≥2时， y= a + 1 a +1 的最小值为1 ③x 2 -mx+1≥0对于x＞0恒成立，则m≤2 ④x≥1的一个充分不必要条件是x=4．A.
1
B.
2
C.
3
D.
4
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