Question:
Which political party did Lin Cheng-chieh belong to in May, 2006?

Context:
 Lin Cheng-chieh is a member of the Democratic Progressive Party from Apr, 1988 to Jun, 1991. 
 Lin Cheng-chieh is a member of the Unionist Party from Jan, 2006 to Jan, 2011. 
 Lin Cheng-chieh is a member of the Kuomintang from Jan, 2016 to Jul, 2019.

Answer:
Unionist Party