The problem of population growth is a problem faced by all countries in the world, especially developed countries. Developed countries generally face low fertility, slow or even negative population growth, which directly affects economic development and ethnic heritage. China is implementing family planning policies. Years later, we also faced similar problems, so China gradually liberalized the second child policy. However, the actual effect is not satisfactory. Some experts pointed out that the unsatisfactory effect of the second child policy is mainly due to social pressure. Which of the following is true and best supports the opinion of the above experts? ['After the second child policy is liberalized, many post-70s couples who want to have a baby have passed their optimal childbearing age.', 'Young couples after the 90s are more willing to live in a two-person world and are not willing to have more children.', 'Because the cost of raising children is too high, many couples are unwilling to have more children.', 'Pollution of the social environment affects the fertility of many young couples.']