以下哪个字典推导式能正确将列表['Python', 'Java', 'C++'] 转换为 {'Python':6, 'Java':4, 'C++':3}（字符串作为键，长度作为值）？（ ）

{len(x): x for x in ['Python', 'Java', 'C++']}

{x: len(x) for x in ['Python', 'Java', 'C++']}

{x: x.length() for x in ['Python', 'Java', 'C++']}

{x: size(x) for x in ['Python', 'Java', 'C++']}