关于摩擦力和功，下列说法正确的是（   ）

Dear Lucy,
   We are back in Garden City now. We had    1   good trip.
   I    2  to collect my photos yesterday. They    3great! I showed these photos    4  my classmates. They all   5  them.
   There is a photo of Ben and Simon  6   the Great Wall. There is one of you and  7in the Forbidden City(紫禁城).
We really enjoyed    8vacation. We were sad(悲伤)to   9   you.
   You must   10and visit us in Garden City at Chinese New Year.
                                                                                                                                                          Love,
                                                                                                                                                          Kitty

将一定质量的NaC1和Na₂SO₄固体混合物完全溶于水后，加入一定质量分数的BaC1₂溶液208克，恰好完全反应，过滤，得到23.3克沉淀，经测定滤液中含氯元素的质量为21.3克(不考虑过滤过程中物质质量的损失)。计算：

(1)所加BaC1₂溶液的溶质质量分数。

(2)原固体混合物中Na₂SO₄的质量。

(3)原固体混合物中NaC1的质量。