Too many libboost_*.lib

I have downloaded boost 1.58.0 (precompiled, x86, VC 12.0) from http://ift.tt/1eGdNQa and installed to C:\local\boost_1_58_0 (I also tried compiled the source code using msvc-12.0 by myself and get the same result.

The problem: I see too many libboost*.lib of the same library, for example

ls -l libboost_math_* returns:

libboost_math_c99f-vc120-mt-1_58.lib
libboost_math_c99f-vc120-mt-gd-1_58.lib
libboost_math_c99f-vc120-mt-s-1_58.lib
libboost_math_c99f-vc120-mt-sgd-1_58.lib
libboost_math_c99f-vc120-s-1_58.lib
libboost_math_c99f-vc120-sgd-1_58.lib
libboost_math_c99l-vc120-mt-1_58.lib
libboost_math_c99l-vc120-mt-gd-1_58.lib
libboost_math_c99l-vc120-mt-s-1_58.lib
libboost_math_c99l-vc120-mt-sgd-1_58.lib
libboost_math_c99l-vc120-s-1_58.lib
libboost_math_c99l-vc120-sgd-1_58.lib
libboost_math_c99-vc120-mt-1_58.lib
libboost_math_c99-vc120-mt-gd-1_58.lib
libboost_math_c99-vc120-mt-s-1_58.lib
libboost_math_c99-vc120-mt-sgd-1_58.lib
libboost_math_c99-vc120-s-1_58.lib
libboost_math_c99-vc120-sgd-1_58.lib
libboost_math_tr1f-vc120-mt-1_58.lib
libboost_math_tr1f-vc120-mt-gd-1_58.lib
libboost_math_tr1f-vc120-mt-s-1_58.lib
libboost_math_tr1f-vc120-mt-sgd-1_58.lib
libboost_math_tr1f-vc120-s-1_58.lib
libboost_math_tr1f-vc120-sgd-1_58.lib
libboost_math_tr1l-vc120-mt-1_58.lib
libboost_math_tr1l-vc120-mt-gd-1_58.lib
libboost_math_tr1l-vc120-mt-s-1_58.lib
libboost_math_tr1l-vc120-mt-sgd-1_58.lib
libboost_math_tr1l-vc120-s-1_58.lib
libboost_math_tr1l-vc120-sgd-1_58.lib
libboost_math_tr1-vc120-mt-1_58.lib
libboost_math_tr1-vc120-mt-gd-1_58.lib
libboost_math_tr1-vc120-mt-s-1_58.lib
libboost_math_tr1-vc120-mt-sgd-1_58.lib
libboost_math_tr1-vc120-s-1_58.lib
libboost_math_tr1-vc120-sgd-1_58.lib

My questions: 1. Why are there so many lib files for one library? (36 files for libboost_math, 4 libboost_atomic, 6 libboost_iostreams and so on) 2. Why are there no single libboost_math.lib, libboost_atomic, ... files? 3. If I want to use boost_math, which library should I choose?