list problem 4 newbie

[manstey]

I can't figure out why my code is not working. I thought I had the list
copied correctly:

Here is my code:

============

a=[[u'HF', []], [u')F', [u'75']], [u'RE', []], [u'C', []]]
b=a[:]

for index in reversed(range(0,len(a)-1)):
   if '75' in b[index][1]:
      b[index][1].remove('75')
      b[index][1].append('99')

print a,'\n',b

============

but when it finishes, I get

[[u'HF', []], [u')F', ['99']], [u'RE', []], [u'C', []]]
[[u'HF', []], [u')F', ['99']], [u'RE', []], [u'C', []]]

instead of the desired:

[[u'HF', []], [u')F', ['75']], [u'RE', []], [u'C', []]]
[[u'HF', []], [u')F', ['99']], [u'RE', []], [u'C', []]]

why is this?