入门第七课的作业 我想让我的回答不等于 b c d 的时候输出...

CYQ

学到入门第七课的作业
我想让我的回答不等于 b c d 的时候输出keep going 应该怎么写呢？
我这么写当输入good的时候会输出 you good  keep going

b = "sb"
c = "good"
d = "great"
print "what do you think i am"
answer = raw_input()

if answer == b:
    print "you sb"
if answer == c:
    print "you good"
if answer == d:
    print "you great"
if answer != b and c and d:
    print "keep going"

crossin先生

answer != b and answer != c and answer != d