Understanding the difference between these very similar substances will be an early challenge for researchers.

An early challenge to their control came in the form of a royal appointment to the rectory.

They have been found around 30 other stars, an earlier challenge to theorists.

The early challenge of mastering the entertainment business had become tedious.

Cooke's early challenges to the law were legal in nature, but rapidly became political.

This was a two part challenge, the first part brought back elements from earlier challenges.

The early challenges were the hilly area and creek to the east.

Another early challenge came from deep suspicions among many (although not all) Protestants.

The earlier challenges, which are still pending, made essentially the same argument.