Why do some Android messages show delivered but not read?

[kyphysics]

Might this be someone who either blocked me or didn't read my latest message on Android phone?

I texted a person we'll call Dave (not his real name). There is a "Read" tag next to these messages with a time-stamp. Pretty nifty of Android to let me know when it was read.

However, we had a contentious exchange via text and I asked him tough questions (that he is dodging I am guessing) and he never responded (for several days). Those texts have a "Delivered" tag, but not the "Read" tag as with the earlier ones (pre-argument).

For those familiar with this phone OS, what could this mean? My guesses were:
a.) He got the texts, but didn't click on them (possibly seeing in the preview something he didn't want to talk about).
b.) He blocked me.
c.) He got the texts, read them, but just doesn't want to respond.

c.) seems unlikely, given other texts have said "Read". . .Does the "Read" tag ever just randomly not show even if someone read your text?

 

[phinds]

I have no idea about android, but in windows outlook when a sender includes a "tell me when read" request, outlook asks you whether you want the sender to see if the email has been read and you can say no.