nums = [5, 12, 7, 9, 21, 7, 14]
# 删除第一次出现的数字7
nums.remove(7) # 只删除第一个7
print("删除第一个7后的列表:", nums)
# 取出第2个到第4个元素(索引1到3)
sub_list = nums[1:4]
print("第2~4个元素组成的列表:", sub_list)
# your code goes here
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