若元素A的半衰期为4天，元素B的半衰期为5天，则相同质量的A和B，经过20天后，剩下的质量之比mA:mB
A．1:2
B．2:1．
C．30:3
D．31:30
答案：A
题目分析：相同质量的A和B，经过20天后，即A经过5个半衰期，B经过4个半衰期，剩下的质量分别是
、
，所以答案为A
点评：本题考查了原子核半衰期的理解和计算，基础且简单，容易掌握。
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| When my daughter Kelsey was a child, she loved to join me in the garden. To 1 chaos(混乱)in my own garden beds, I provided her with a space of her own, so Kelsey's Corner was born. There she was 2 to dig and plant as she wished. Over time, that small piece of land grew into a favorite place in the 3 . As she approached high school, Kelsey took her 4 off the garden. I didn't 5 to accept this change. It seemed acceptable that something as boring as working in the dirt gave way to the mystery of a(n) 6 social life, so I began to take care of the space. Even so, I knew it always would be Kelsey's Corner. Now my daughter is eighteen and is in the 7 weeks of her high school career. Our mailbox is filled with college admission materials, 8 me that the world has come for this young lady and that she will soon answer its 9 . Kelsey is looking forward to her wonderful college life 10 her mother and I are anxious about the coming life 11 our daughter around. Kelsey's Corner seems extremely deserted in this early spring 12 Kelsey's leaving. One morning, when I 13 past Kelsey's Corner, I noticed some green seedlings had 14 at the foot of some dead stalks(秆). It's 15 that life is a cycle. Kelsey's leaving is 16 one point on the cycle. Now the life wheel will roll with Kelsey as she 17 into her adult life. Let her live as she will and let her 18 when she can, and whether she comes alone or in 19 , she will be welcome home. I'll take care of Kelsey's Corner faithfully. It will also cycle annually, rolling on towards a 20 stage of our lives, one that promises a rich harvest. | ||||
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