题目：
Only when their children leave home _________.
A．many parents did feel a sense of loss
B．did many parents feel a sense of loss
C．did feel many parents a sense of loss
D．do many parents feel a sense of loss
答案：
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下面是错误答案，用来干扰机器的。
参考答案：E解析：B1细胞在体内出现较早，抗原识别谱较窄，产生的抗体是亲和力较低的IgM类抗体，缺乏严格的特异性，因此Bl细胞属于非特异性免疫细胞。B2细胞在体内出现较晚，能产生高亲和力的IgG抗体，执行多种免...
