题目选项分析

The Cygnus spacecraft was grounded last week ahead of its launch date

because massive coronal ejections from the Sun had raised the radioactivity to such levels that its avionics systems might malfunction

The Cygnus spacecraft was grounded last week ahead of its launch date

because of massive coronal ejections from the Sun, which had raised the radioactivity to levels such that its avionics systems would malfunction