请修改列表生成式，通过添加if语句保证列表生成式能正确地执行：
L1 = ['Hello', 'World', 18, 'Apple', None] L2 = [s.lower() for s in L1 if isinstance(s,str)] L2 = [s.lower() for s in L2] print(L2)
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我很有素质
请修改列表生成式，通过添加if语句保证列表生成式能正确地执行：
L1 = ['Hello', 'World', 18, 'Apple', None] L2 = [s.lower() for s in L1 if isinstance(s,str)] L2 = [s.lower() for s in L2] print(L2)