error shows when install requirement.txt

[JulyUp]

hello, First I use virtualenv. And when I do the command 'pip install -r requirements.txt',

it show error
`/home/xxx/work/web/flask-web/weblog/venv/build/lxml/src/lxml/includes/etree_defs.h:14:31: fatal error: libxml/xmlversion.h: No such file or directory

#include "libxml/xmlversion.h"

                           ^

compilation terminated.

error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

---

Cleaning up...
Command /home/xxx/work/web/flask-web/weblog/venv/bin/python -c "import setuptools, tokenize;file='/home/xxx/work/web/flask-web/weblog/venv/build/lxml/setup.py';exec(compile(getattr(tokenize, 'open', open)(file).read().replace('\r\n', '\n'), file, 'exec'))" install --record /tmp/pip-Sb75vo-record/install-record.txt --single-version-externally-managed --compile --install-headers /home/xxx/work/web/flask-web/weblog/venv/include/site/python2.7 failed with error code 1 in /home/xxx/work/web/flask-web/weblog/venv/build/lxml
Storing debug log for failure in /home/congleng/.pip/pip.log`

how to solve it?