Fact 1: Trees have woody stems covered with bark.

Fact 2: On the outside is a protective skin of bark made mainly of cork.

Given the two facts above, answer the question "Trees have woody stems covered with what?" with the following options: 
- heat energy
 - thin skin
 - protective skin
 - skinny cells
 - white skin
 - Chlorophyll
 - Organic compounds
 - looseness


 

   protective skin      