如果字符串以某个字符开头，则替换该字符串

def replace_if_starts_with(string, start_char, replacement):
    if string.startswith(start_char):
        new_string = string.replace(start_char, replacement)
        return new_string
    else:
        return string

# 示例用法
original_string = "apple is a fruit"
start_char = "a"
replacement = "An"
new_string = replace_if_starts_with(original_string, start_char, replacement)
print(new_string)