Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: Saudi Arabia will not rule out building nuclear weapons to defend itself against Iran, its ambassador to the United States has said. Adel Al-Jubeir told CNN the 'kingdom of Saudi Arabia will take whatever measures are necessary to protect its security'. Iranian officials are due to meet with world leaders to negotiate a relief in sanctions against the country in return for Tehran curbing its nuclear programme. But Al-Jubeir claimed that other countries in the Middle East are 'concerned' about the prospect of Iran developing nuclear weapons. The Gulf nation and its allies today launched airstrikes against Houthi rebels inside Yemen for a second day straight - in a move it claims protects its southern border from attack.Saudi Ambassador to US said they will take measures to protect countryHe said Middle East countries worried Iran will develop nuclear weaponsIranian officials are due to meet with world leaders for 'nuclear talks' soonSaudi-led airstrikes targeted Iranian-backed rebels in Yemen again todayThe Gulf nation wants to avoid potential attack from its southern border 
 Question:Some experts believe the escalating conflict in Yemen (pictured) is turning into a proxy war between the _'s and Yemen.

Saudi Arabia