After nginx reverse proxy configuration, home page 404

Local agent, develop online interface. Nginx reverse proxy is used. The result is that the home page can only be accessed with index.html, and the direct access slash / is 404.

worker_processes  1;
events {
    worker_connections  1024;
}
http {
    include       mime.types;
    default_type  application/octet-stream;
    sendfile      on;
    keepalive_timeout 10;
    upstream serverip {
        server xx.xx.xx.xx;
    }

    server {
        listen  80;
        server_name  a.com;
        
        location =/ {
            add_header X-Frame-Options SAMEORIGIN;
            index index.html;
        }

        location ~* \.(html|htm|gif|jpg|jpeg|bmp|png|ico|txt|js|css|swf|woff|woff2|ttf|json|svg|cur|vue|otf|eot)$ {
            charset     utf-8;
            root        D:/WWW/app;
            expires     3d;
        }
        
        location / {
            proxy_pass   http://serverip;
        }
    }
}

hosts:

127.0.0.1 a.com

if I change the configuration to the following, I can access it in both ways: http://a.com/ and http : / / a.com/index.html

location =/ {
    add_header X-Frame-Options SAMEORIGIN;
    root        D:/WWW/app;
    index index.html;
}

location ~* \.(html|htm|gif|jpg|jpeg|bmp|png|ico|txt|js|css|swf|woff|woff2|ttf|json|svg|cur|vue|otf|eot)$ {
    charset     utf-8;
    root        D:/WWW/app;
    expires     3d;
}

take a look at the log
D:WWWappnginx/html/index.html "is not found (3: The system cannot find the path specified),

explain here, my project directory:
www/app/index.html
www/app/nginx/*

I want to ask, do I have to configure two root? Can I configure two root?

Mar.20,2021

make sure you configure root, or you won't find the file at all

MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3be52-2b818.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3be52-2b818.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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