Why can I see the Windows 10 drive in Windows 7

[kwfine]

Hi all,
Recently I bought a new SSD.
I installed Windows 7 first in the first partition (40GB), and then Windows 10 in the other partition (80GB).
Then I was learning how to use the new features in Windows 10 without switching to Windows 7 during the Dual Boot.
When I was using Windows 10, I could only see one Drive (C), there was not any other parition.
But some strange things happened when I chose to use Windows 7 in the Dual Boot.
After I logged on Windows 7 and double clicked My Computer, I could see there were two main drives (C and D drives)
The C drive surely was for Windows 7 because of its volume (40GB), and D Drive was 80GB. When I clicked to see what's iniside the D drive, I could see a Windows folder and a Windows.old folder, and some other folders.

I have no idea why this happened.
Could you help me please?

Thank you.

[Geek-9pm]

Just leave it alone. Unless it creates a problem.
When you install a new version of windows, the older version is saved.
You have lots of drive space. Right? If so, do nothing.

[kwfine]

Just leave it alone. Unless it creates a problem.
When you install a new version of windows, the older version is saved.
You have lots of drive space. Right? If so, do nothing.

Thank you for the quick help, Geek-9pm.
I was so worried about it because that was my first time learning how to make a Dual-Boot.

[Allan]

The system will always designate the drive to which you boot (the system drive) as C: drive. If you installed W7 on c: and then installed W10 on d:, when you boot to W10 the system will change the designations so that it boots to what is now c: drive, but was previously d: drive when you were in W7.

[patio]

The Win7 install sees 10 because 7 is on the C:/1st partition...
When 10 was installed it is on another partition and does not recognise C: because in fact the PC is faked into thinking the 10 partition is in fact C:...when it's not.