A．条件(1)充分，但条件(2)不充分．
B．条件(2)充分，但条件(1)不充分．
C．条件(1)和(2)单独都不充分，但条件(1)和条件(2)联合起来充分．
D．条件(1)充分，条件(2)也充分．
E．条件(1)和(2)单独都不充分，条件(1)和条件(2)联合起来也不充分．

x³+y³+3xy=1．
(1) x+y=1 (2) [*]

Optimism is the art of always seeing the glass half full (1) half empty, finding something positive in even the most (2) of situations and last but not least the ability to expect the best even if the chances are (3) . It enables you to try new things, deal with failure as a learning experience (4) a stopping point. In general optimism gives you a reason to live. The world would be a better place if it was full of optimism, (5) unfortunately the trend in today’s youth is (6) the opposite way. Today it is (7) to be pessimistic. One could spend hours discussing how we arrived at this point, (8) our youth refuses to smell the roses and instead prefers to (9) themselves on their own pessimistic (10) .

Optimism is not an (11) gift but rather a learned behavior (12) . Like most behaviors optimism is best learned by watching examples, (13) you. The first step in creating a generation of people with a positive outlook on life, is to (14) life on the bright side yourself. (15) it would be wrong to (16) your child from all negativity, it is important that you are not the (17) of doubts and worries. Finding something positive in a negative situation needs to be (18) Show how to make the best of things by always (19) the positive, by encouraging your child after a(n) (20) and by explaining over and over that failure is just a part of progress.

13（）

A.primarily

B.frequently

C.namely

D.normally