钟面上12点30分，时针与分针的夹角是____度．

下列关于青春期的叙述，正确的是（   ）

①1.0L1.0mo1•L^﹣1的NaAlO₂水溶液中含有的氧原子数为2N_A

②过氧化钠与水反应时，生成0.1mol氧气转移的电子数为0.2N_A

③密闭容器中2molNO与1molO₂充分反应，产物的分子数为2N_A

④.235g核素 ${}_{92}^{235}$ U发生裂变反应： ${}_{92}^{235}$ U+ ${}_0^1$ n $\stackrel{裂变}{\to }$ ${}_{38}^{90}$ Sr+ ${}_{54}^{136}$ Xe+10 ${}_0^1$ n，净产生的中子（ ${}_0^1$ n）数为10N_A

⑤氢氧燃料电池正极消耗22.4L（标准状况）气体时，电路中通过的电子数目为2N_A

⑥铜粉溶于1L0.5mol/L稀硝酸中，当生成标况下2.24LNO时，溶液中的氮原子数为0.4N_A个

⑦标准状况下，11.2升氯气溶于水，转移电子数为0.5N_A

⑧12g石墨和C₆₀的混合物中质子总数一定为6N_A

⑨25℃时，1L mol•L^﹣1FeCl₃溶液中含有0.1N_A个Fe（OH）₃胶体粒子．

    On March 14, maths and science lovers around the world celebrated a special day: Pi Day.

    Pi is____ to3. 14, but the number goes on endlessly. It is sometimes written in Greek,π. With the help of computers, mathematicians have been able to ____ pi out to over trillion (万亿) decimal places, but there is still no____ to the number. This makes pi puzzling, even for the most famous scientists and mathematicians.

    Pi Day is celebrated around the world on March 14, since how we write this date, 3/14,looks just like the number pi.

    For some people, the appeal (吸引力) of Pi Day goes far beyond maths and science. Pi shows up____popular culture. You can see it in movies, comics, music and more. "In modern movies, any time the filmmaker wants to evoke (产生) a sense of mystery, often the symbol pi is used, "says David Blatner, Jewish American writer of The Joy of Pi.