why I must to specify the version for che child project?

[陈思淼]

In my parent Project the pom is:
<groupId>com.mycompany.trademanager</groupId>
  <artifactId>all</artifactId>
  <packaging>pom</packaging>
  <version>1.0-SNAPSHOT</version>
  <modules>
   <module>common</module>

  </modules>
  <dependencies>
  <dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.3.1</version>
<scope>test</scope>
</dependency>
</dependencies>
 In my child project the pom is:
  <parent>
   <groupId>com. mycompany.trademanager</groupId>
<artifactId>all</artifactId>
<version>1.0-SNAPSHOT</version>
</parent>

  <groupId>com.mycompany.trademanager</groupId>
  <artifactId>common</artifactId>
  <packaging>jar</packaging>
  <version>1.0-SNAPSHOT</version>
  <build>
  </build>
  <dependencies>
  <dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.3.1</version>
</dependency>
<dependency>
</dependencies>
I must specify theversion explicitly,If not ,the Error occur:[0]
 'dependencies.dependency.version' is missing for junit:junit