What is incorrect in the following code? prefix = "auto" root = "mobile" word = concat_strings (prefix, root) def concat_strings(s1, s2): Returns a single string which is the concatenation of the two strings provided. combined_string = s1+52 return combined_string O The function call of concat_strings() precedes the function definition. O The + operator cannot be used on strings The word "string" cannot be used in variable names or function names. O The return statement requires parentheses i.e. return(combined_string)