Hello I'm just wondering what PL/SQL Lexer error: Unterminated string means? I've been researching it but I cant seem to get it

Question

这是我的代码

DECLARE 
d_country_name VARCHAR2 (50); 
d_median_age NUMBER(6,2); 
BEGIN 
SELECT country_name, median_age INTO d_country_name, d_median_age 
FROM wf_countries 
WHERE country_name = ‘United States of America'); 
DBMS_OUTPUT.PUT_LINE(' The median age in '||d_country_name||' is '||d_median_age||'.'); 
END; 

但是它说

ORA-20001：第 8 行错误，col85：PL/SQL 词法分析器错误：未终止的字符串

我看不出有什么问题，先谢谢了！

Answer 1

America' 之后的第一个引号和额外的括号似乎不正确。