Questions

France joined the Triple Entente because it needed protection from Germany.

France felt very heavily the aggression from Germany (a country which it shared a border with), especially over the tension Alsace and Lorraine and the constant switching over of these two counties.

It was also worried about the rapid military advancement in Germany and France wanted to ally itself with Russia and Great Britain.

In this position, the head light will be at the principal focus so that the rays of light falling on the cancave mirror will be rendered parallel

C++ is not an Operating System.

An Operating System is a collection of system programs that manages all the other programs application programs in a computer as well as the allocation and use of hardware resources such as the CPU, Memory and the Hard Disk Drive.

It acts as an interface between the hardware and the user level program.

It controls and facilitates the overall operation of a computer. 

Here

UNIX : UNIX is a popular multi-user, multitasking operating system (OS) developed at Bell Labs.

Windows NT : Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993.

DOS : DOS is Disk Operating System. The term DOS can refer to any operating system.

C++ : C++ is a programming language and computing platform.

Hence, C++ is not an Operating System.