大全68，再次讨论~谢谢~

1.        After the Colonial period’s 50 percent mortality rate, life expectancy improved for children, but as late as the nineteenth century about one child in three died before reaching the age of six.

(A) After the Colonial period’s 50 percent mortality rate, life expectancy improved for children, but

(B) Even though children’s life expectancy, which improved over the Colonial period’s 50 percent mortality rate,

(C) Although life expectancy for children improved after the Colonial period, during which the mortality rate was 50 percent,

(D) While there was an improvement in life expectancy for children after the 50 percent mortality rate of the Colonial period, still（C）

(E) Despite children’s life expectancy improvement from the Colonial period’s 50 percent mortality rate,

why not D, wht is the usage for word ' while'? cheers~