dse數學問題

以下問題都已知答案,但我不明白為何是這些答案,
希望數學勁人幫幫忙,詳細解說一下,謝謝!>,<"

Q1)
http://upload.lsforum.net/users/public/w2876P290(Q15)t192.gif
ans. is C, but why B is not??

Q2)
http://upload.lsforum.net/users/public/z16858P302(Q43)t192.gif
ans. is A, but why B is not??

Q3)
Emily puts 6 pens into 3 different pencil cases.
In how many ways can she put the pens?
ans. is 729.

Q4)
8 boys and 5 girls are nominated for 4 different posts in a student association.
How many ways are there to select the students?
ans. is 17160.

Q5)
There are 8 bags on a table. 5 of them are empty and each of the other bags contains one ball in it. Andy puts a ball into a bag at random. If a bag is chosen afterwards, the probability of getting an empty bag is
ans. is 35/64.

Q6)
http://upload.lsforum.net/users/public/l20875P70(Q40)k192.gif
ans. is A.

Q7)
http://upload.lsforum.net/users/public/c30105P70(Q2)w192.gif
ans. is A.

Q8)
Which of the following figures cannot be used to tessellate a plane completely?
A. Parallelogram
B. Isosceles triangle
C. Regular hexagon
D. Regular pentagon
ans. is D.

Q9)
http://upload.lsforum.net/users/public/e9885P73(Q20)a192.gif
ans. is D.

Q10)
http://upload.lsforum.net/users/public/w15768P128(Q20)h192.gif
ans. is C.

Q11)
In triangle ABC, the coordinates of the mid-points of the three sides are (1,5),(2,4) and (4,6) respectively. Which of the following points is a vertex of the triangle?
A. (-1,4)
B. (0,3)
C. (3,7)
D. (5,6)
ans. is C.

Q12)
http://upload.lsforum.net/users/public/i53040P189(Q17)j192.gif
ans. is D.