解释生物进化的原因，目前人们普遍接受的是达尔文的自然选择学说，以下关键词能正确概述自然选择学说内容的一组是（    ）

①自然发生  ②物种不变  ③过度繁殖  ④环境污染  ⑤生存斗争  ⑥遗传、变异  ⑦适者生存

某课外实验小组设计的下列实验合理的是

阅读下面短文，在空白处填入一个适当的词，或填入括号中所给单词的正确形式。    I had my summer all planned out．I was going to enjoy the city． However, my parents had ____ different idea．They said l would be going to my grandparents' farm ____ a whole month．I felt sad．What could ____ (possible)be fun to do in the country?
    It was very hot when we arrived at the farm．So Grandpa took ____ (I)to a creek(小溪)．It felt great to jump into the cool，clear water．I ____ (begin)to think staying in the country might be ____ (interesting)than I had thought．
    The next day Grandma gave me a basket and sent me out ____(pick)strawberries．The berries were so good that a lot of them had disappeared ____ they reached my basket．
    I didn't get to visit a zoo， but l got an up-close look at pigs，cows，and ____ (chicken)．I fed the goats，milked the cows，and collected fresh eggs．
    My summer wasn't what I had expected，____ I loved it! I was sad that I had to leave．Now, I still love living in the city，but I learned that trying new things can be a lot of fun，too!

读下图，完成下列问题。

我国人口集中、工农业发达、内河水运最便利的地区是（  ）