der which NUnit is currently 
running, then that runtime is used.

<li><p>If the assembly was built for a different runtime version and that version 
is available, NUnit uses it, running out of process if necessary. 

<li><p>If the assembly was built for a different runtime version, which is not
available, then the result depends on whether the build version is earlier or
later than the current version. If earlier, the test will be run using the
same runtime version under which NUnit is running. If later, t