[R] number of individuals where X=0 during all periods (longitudinal data)

[najuzz]

#Hi guys, 

#I would like to count the number of individuals that receive X=0 troughout
their observational period.
#example dataset:

ID<-c(1,1,1,1,2,2,3,3,3)
X<-c(0,1,2,1,0,0,0,0,0)
Time<-c(1,2,3,4,1,2,1,2,3)
Test<-data.frame(ID,X,Time)

# Individuals 2 and 3 have x=0 during all their periods. The count should
hence equal to two. I simply have 
# no clue how R could solve this for me. As an addon, I would also like to
know the number of individuals  #that report X=0 during all periods plus
have at least 3 weeks of observations. The answer would be one in #this
sample datset.

#Thank you



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